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An illustration of cannabis plants

Cannabis has long been used for medicinal as well as recreational purposes. Image source

Few topics spark as much debate on this blog and on our Facebook page than cannabis.

So we thought we’d take a look at the common questions raised about the evidence and research into cannabis, cannabinoids (the active chemicals found in the plant and elsewhere) and cancer, and address some of the wider issues that crop up in this debate.

We’ve broken it down under a number of headings:

This post is long, but can be summarised by saying that at the moment there isn’t enough reliable evidence to prove that cannabinoids – whether natural or synthetic – can effectively treat cancer in patients, although research is ongoing around the world.

Read on to get the full picture.

What are cannabinoids and how do they work?

Cannabinoids” is a blanket term covering a family of complex chemicals (both natural and man-made) that lock on to cannabinoid receptors – protein molecules on the surface of cells.

Humans have been using cannabis plants for medicinal and recreational purposes for thousands of years, but cannabinoids themselves were first purified from cannabis plants in the 1940s. The structure of the main active ingredient of cannabis plants – delta-9 tetrahydrocannabinol (THC) – was discovered in the 60s. It wasn’t until the late 1980s that researchers found the first cannabinoid receptor, followed shortly by the discovery that we create cannabinoid-like chemicals within our own bodies, known as endocannabinoids.

The CB1 and CB2 receptors

The CB1 and CB2 receptors. Image source

We have two different types of cannabinoid receptor, CB1 and CB2, which are found in different locations and do different things. CB1 is mostly found on cells in the nervous system, including certain areas of the brain and the ends of nerves throughout the body, while CB2 receptors are mostly found in cells from the immune system. Because of their location in the brain, it’s thought that CB1 receptors are responsible for the infamous ‘high’ (known as psychoactive effects) resulting from using cannabis.

Over the past couple of decades scientists have found that endocannabinoids and cannabinoid receptors are involved in a vast array of functions in our bodies, including helping to control brain and nerve activity (including memory and pain), energy metabolism, heart function, the immune system and even reproduction. Because of this molecular multitasking, they’re implicated in a huge range of illnesses, from cancer to neurodegenerative diseases.

Can cannabinoids treat cancer?

There is no doubt that cannabinoids – both natural and synthetic – are interesting biological molecules. Hundreds of scientists around the world are investigating their potential in cancer and other diseases – as well as the harms they can cause – brought together under the blanket organisation The International Cannabinoid Research Society.

Researchers first looked at the anticancer properties of cannabinoids back in the 1970s, and many hundreds of scientific papers looking at cannabinoids and cancer have been published since then. This Wellcome Witness seminar is also fascinating reading for aficionados of the history of medical cannabis, including the scientific, political and legal twists. [Updated KA 26/03/14]

But claims that this body of preclinical research is solid “proof” that cannabis or cannabinoids can cure cancer is highly misleading to patients and their families, and builds a false picture of the state of progress in this area. For example, we’ve taken a look at more than 30 scientific papers that are often claimed to “prove” that cannabis cures various types of cancer. [Updated KA 21/07/14]

Let’s take a closer look at the evidence.

Lab research

Virtually all the scientific research investigating whether cannabinoids can treat cancer has been done using cancer cells grown in the lab or animal models. It’s important to be cautious when extrapolating these results up to real live patients, who tend to be a lot more complex than a Petri dish or a mouse.

A researcher with some cells in a Petri dish

Virtually all the research into cannabinoids and cancer so far has been done in the lab.

Through many detailed experiments, handily summarised in this recent article in the journal Nature Reviews Cancer, scientists have discovered that various cannabinoids (both natural and synthetic) have a wide range of effects in the lab, including:

  • Triggering cell death, through a mechanism called apoptosis
  • Stopping cells from dividing
  • Preventing new blood vessels from growing into tumours
  • Reducing the chances of cancer cells spreading through the body, by stopping cells from moving or invading neighbouring tissue
  • Speeding up the cell’s internal ‘waste disposal machine’ – a process known as autophagy – which can lead to cell death

All these effects are thought to be caused by cannabinoids locking onto the CB1 and CB2 cannabinoid receptors. It also looks like cannabinoids can exert effects on cancer cells that don’t involve cannabinoid receptors, although it isn’t yet clear exactly what’s going on there.

So far, the best results in the lab or animal models have come from using a combination of highly purified THC and cannabidiol (CBD), a cannabinoid found in cannabis plants that counteracts the psychoactive effects of THC. But researchers have also found positive results using synthetic cannabinoids, such as a molecule called JWH-133.

It’s not all good news though, as there’s also evidence that cannabinoids may also have undesirable effects on cancer.

For example, some researchers have found that although high doses of THC can kill cancer cells, they also harm crucial blood vessel cells, although this may help their anti-cancer effect by preventing blood vessels growing into a tumour. And under some circumstances, cannabinoids can actually encourage cancer cells to grow, or have different effects depending on the dosage and levels of cannabinoid receptors present on the cancer cells. [Edited for clarity and to add reference - KA 27/07/12]

Others have discovered that activating CB2 receptors may actually interfere with the ability of the immune system to recognise and destroy tumour cells, although some scientists have found that certain synthetic cannabinoids may enhance immune defences against cancer.

Furthermore, cancer cells can develop resistance to cannabinoids and start growing again, although this can be got round by blocking a certain molecular pathway in the cells known as ALK.

And yet more research suggests that combining cannabinoids with other chemotherapy drugs may be a much more effective approach. This idea is supported by lab experiments combining cannabinoids with other drugs including gemcitabine and temozolomide.

As might be expected, whenever research about cannabis or cannabinoids hits the news there is a lot of  interest on social media. But often it turns out that the hype don’t realistically reflect the work. For example, this study from researchers at the University of East Anglia was done using cancer cells grown in the lab or transplanted into mice, to try and understand why different levels of purified THC seem to have different effects on cancer cells – something that has been noticed from previous experiments on cannabinoids and cancer cells.

The researchers found that THC seems to work through two different receptor molecules coming together, and that high doses slow cancer cells growth while low doses don’t. So they think that designing drugs that make sure the receptors come together in the right way to kill cancer cells could be a good way to harness the potential power of cannabinoids to treat cancer in a much more effective and targeted way.

But while it’s an interesting scientific paper and helps to shed light on the molecular “nuts and bolts” that underpin how some cancer cells may respond to cannabinoids, and could point to ways to make cannabinoid drugs more effective in the future, it certainly doesn’t tell us that cannabis can effectively treat cancer in patients at the moment.

Clinical research

But that’s the lab – what about clinical research involving people with cancer? Results have been published from only one clinical trial testing whether cannabinoids can treat cancer in patients, led by Dr Manuel Guzman and his team in Spain. Nine people with advanced, terminal glioblastoma multiforme – an aggressive brain tumour – were given highly purified THC through a tube directly into their brain.

Eight people’s cancers showed some kind of response to the treatment, and one didn’t respond at all. All the patients died within a year, as might be expected for people with cancer this advanced.

The results from this study show that THC given in this way is safe and doesn’t seem to cause significant side effects. But because this was an early stage trial, without a control group, it’s impossible to say whether THC helped to extend their lives. And while it’s certainly not a cure,  the trial results suggest that cannabinoids are worth pursuing in clinical trials.

There is also a published case report of a 14-year old girl from Canada who was treated with cannabis extracts (also referred to as “hemp oil”), but there is limited information that can be obtained from a single case treated with a varied mixture of cannabinoids. More published examples with detailed data are needed in order to draw a fuller picture of what’s going on. [Updated 26/03/14, KA] 

Unverified anecdotes about ‘cures’ do little to help progress towards more effective treatments for patients on a wider scale - even if they do get published in newspapers, they aren’t strong scientific evidence. In order to build a solid evidence base that might support future applications for funding or clinical trials it’s important to gather together detailed information about individual cases.

Here in the UK, Dr Wai Liu at St George’s University is researching cannabis and cannabinoids for treating cancer (we are not currently funding his work). You can read about some of his recent work here.  He is happy to collect individual stories from UK patients, and can be contacted by email. In the US, the Office of Cancer Complementary and Alternative Medicine gathers similar stories for their Best Case Series.  [Updated KA 21/07/14]

A handful of other clinical trials of cannabinoids are currently being set up. We are helping to support the only two UK trials of cannabinoids for treating cancer, through our Experimental Cancer Medicine Centre (ECMC) Network funded by Cancer Research UK and the devolved Departments of Health. One early-stage trial is testing a synthetic cannabinoid called dexanabinol in patients with advanced cancer, and the other is an early-stage trial testing a cannabis extract called Sativex for treating people with glioblastoma multiforme brain tumours. [Edited to add more information about the trials - KA 22/08/12, KA 24/03/14]

Unanswered questions

There are still a lot of unanswered questions around the potential for using cannabinoids to treat cancer.

Cannabis extract

An antique bottle of cannabis extract. Image source

The biggest issue is that there isn’t enough evidence to show that they can treat cancer in people, although research is still ongoing around the world.

And it’s not clear which type of cannabinoid – either natural or synthetic – might be most effective, what kind of doses might be needed, or which types of cancer might respond best to them. So far there have been intriguing results from lab experiments with prostate, breast, lung cancer, skin, bone and pancreatic cancers, glioma brain tumours and lymphoma. But the take-home message is that different cannabinoids seem to have different effects on various cancer types, so they are far from being a ‘universal’ treatment.

Most research has been focused on THC, which occurs naturally in cannabis plants, but researchers have found that different cannabinoids seem to work better or worse different types of cancer cells. Lab experiments have shown promising results with THC on brain tumour and prostate cancer cells, while CBD seems to work well on breast cancer cells.

Then there’s the problem of the psychoactive effects of THC, particularly at high doses, although this can be counteracted by giving it together with CBD. Because of this problem, synthetic cannabinoids that don’t have these effects might be more useful in the long term.

There are also big questions around the best way to actually get the drugs into tumours. Because of their chemical makeup, cannabinoids don’t dissolve easily in water and don’t travel very far in our tissues. This makes it hard to get them deep into a tumour, or even just deliver them into the bloodstream in consistently high enough doses to have an effect.

The clinical trial led by Dr Guzman in Spain involved directly injecting cannabinoids into patients’ brains through a small tube. This isn’t an ideal method as it’s very invasive and carries a risk of infection, so researchers are investigating other delivery methods such as tablets, oil injections, mouth sprays or even microspheres.

We also don’t know whether cannabinoids will help to boost or counteract the effects of chemotherapy, nor which combinations of drugs might be good to try. And there are currently no biological markers to help doctors identify who might benefit from cannabinoids and who might not – remember that one patient on the brain tumour trial failed to respond to THC at all.

None of these issues are deal-breakers, but these questions need answering if there’s any hope of using cannabinoids to effectively and safely treat cancer patients.

It’s worth remembering that there are hundreds of exciting potential cancer drugs being developed and tested in university, charity and industry labs all over the world – cannabinoids are merely a small part of a much larger picture.

Most of these compounds will never make it into the clinic to treat patients for a huge range of reasons including toxicity, lack of effectiveness, unacceptable side effects, or difficulty of delivering the drug to tumours.

Without doing rigorous scientific research, we will never sift the ‘hits’ from the ‘misses’. If cannabinoids are ever to get into clinical use, they need to overcome these hurdles and prove they have benefits over existing cancer treatments.

Can cannabis prevent or cause cancer?

So that’s a brief look at cannabinoids to treat cancer. But can they stop the disease from developing? Or could they play a role in causing cancer?

Someone smoking a cannabis joint

There’s controversy around the health risks of cannabis. Image source

In experiments with mice, animals given very high doses of purified THC seemed to have a lower risk of developing cancer, and there has been some research suggesting that endocannabinoids (cannabinoids produced by the body) can suppress tumour growth. But there’s no solid scientific evidence at the moment to show that cannabinoids or cannabis can cut the risk of cancer in people.

When it comes to finding out whether cannabis can cause cancer, the evidence is a lot murkier. This is mainly because most people who use cannabis smoke it mixed with tobacco, a substance that definitely does cause cancer.

This complex issue recently hit the headlines when the British Lung Foundation released a study suggesting that the cancer risks of cannabis had been underestimated, although this has been questioned by some experts including Professor David Nutt.

What about controlling cancer symptoms such as pain or sickness?

Although there’s a lack of data showing that cannabinoids can effectively treat cancer, there is good evidence that these molecules may be beneficial in other ways.

As far back as the 1980s, cannabinoid-based drugs – including dronabinol (synthetic THC) and nabilone – were used to help reduce nausea and vomiting caused by chemotherapy. But there are now safer and more effective alternatives and cannabinoids tend to only be used where other approaches fail.

In some parts of the world – including the Netherlands – medical use of marijuana has been legalised for palliative use (relieving pain and symptoms), including cancer pain. For example, Dutch patients can obtain standardised, medicinal-grade cannabis from their doctor, and medicinal cannabis is available in many states in the US.

But one of the problems of using herbal cannabis is about dosage – smoking it or taking it in the form of tea often provides a variable dose, which may make it difficult for patients to monitor their intake. So researchers are turning to alternative dosing methods, such as mouth sprays, which deliver a reliable and regulated dose.

Large-scale clinical trials are currently running in the UK testing whether a mouth spray called Sativex (nabiximols) – a highly purified pharmaceutical-grade extract of cannabis containing THC and CDB – can help to control severe cancer pain that doesn’t respond to other drugs.

There may also be potential for the use of cannabinoids in combating the loss of appetite and wasting experienced by some people with cancer, although a clinical trial comparing appetite in groups of cancer patients given cannabis extract, THC and a placebo didn’t find a difference between the treatments.

Is Cancer Research UK investigating cannabinoids?

We want to see safe, reliable and effective treatments become available for patients as quickly as possible. We receive no government funding for our research, and it is all paid for by the generosity of the public.  This is obviously not a bottomless purse, and we do not have financial reserves to draw on.

Because of this limitation, we can only fund the very best research proposals that come to us that will bring benefits to people with cancer.  We’ve previously written in detail about how we fund research projects.

Cancer Research UK has funded research into cannabinoids, notably the work of Professor Chris Paraskeva in Bristol investigating the properties of cannabinoids as part of his research into the prevention and treatment of bowel cancer. He has published a number of papers detailing lab experiments looking at endocannabinoids as well as THC, and written an interesting review looking at the potential of cannabinoids for treating bowel cancer.

Our funding committees have previously received other applications from researchers who want to investigate cannabinoids that have failed to reach our high standards for funding. If we receive future proposals that do meet these stringent requirements, then there is no reason why they would not be funded – assuming we have the money available to do so.

We support the only two UK clinical trials of cannabinoids for treating cancer through our national network of Experimental Cancer Medicine Centres, funded by Cancer Research UK and the devolved Departments of Health. One is an early-stage trial testing a synthetic cannabinoid called dexanabinol for people with advanced cancer, the other is an early-stage trial testing a drug called Sativex (an extract from cannabis plants) for people with glioblastoma multiforme brain tumours[Added 22/08/12 - KA, Updated KA 25/03/14]

“It’s natural so it must be better, right?”

There’s no doubt that the natural world is a treasure trove of biologically useful compounds. But whole plants or other organisms are a complex mix of hundreds of chemicals (not all of which may be beneficial) and contains low or variable levels of active ingredients. This makes it difficult to give accurate doses and runs the risk of toxic side effects.

Foxgloves

Foxgloves – a source of medically useful chemicals. Image source

For example, foxgloves (Digitalis) are a useful source of chemicals called cardiac glycosides, first purified in 1785 – a date widely considered to be the beginning of modern drug-based medicine. These drugs are now used to treat many thousands of people around the world with heart failure and other cardiac problems. But the entire plant itself is highly toxic, and eating just a small amount can kill.

As another example, although the antibiotic penicillin was first discovered in a fungus, it doesn’t mean that someone should munch some mould when suffering an infection. In fact, the bug-beating powers of ‘natural’ penicillin are confined to a relatively small range of bacteria, and chemists have subsequently developed a wider range of life-saving antibiotics based on the drug’s structure.

Aspirin is another old drug, first discovered in the form of salicylic acid in white willow bark. But this naturally-occurring chemical causes severe stomach irritation, which led to the German company Bayer developing an alternative version – acetylsalicylic acid – which was kinder to the tummy. Aspirin is now arguably one of the most successful drugs of all time, and is still being investigated for its potential in preventing or even treating cancer.

Numerous potent cancer drugs have also been developed in this way – purifying a natural compound then improving it and testing it to create a beneficial drug – including taxol (originally from yew leaves); vincristine and vinblastine (from rosy periwinkles); camptothecin (from the Chinese Xi Shu tree); colchicine (from crocuses); and etoposide (from the May Apple). And we recently wrote about a clinical trial being run by our scientists to test whether curcumin, a purified chemical from the curry spice turmeric, could help treat people with advanced bowel cancer.

But it bears repeating that the fact that these purified drugs in controlled, high doses can treat cancer doesn’t mean that the original plant (or a simple extract) will have the same effect.  So although cannabis contains certain cannabinoids, it doesn’t automatically follow that cannabis itself can treat cancer.

As we said above, there is no good evidence that natural cannabinoids, at the doses present in simple cannabis preparations, can treat cancer in patients. It’s also completely unknown whether there may be any other chemicals in ‘street’ cannabis extracts that could be harmful to patients or even encourage tumour growth.

“Have you seen this video? This guy says cannabis cures cancer!”

There is a strong and persistent presence on the internet arguing that cannabis can cure cancer. For example, there are numerous videos and unverified anecdotes claiming that people have been completely cured of cancer with cannabis, hemp/cannabis oil or other cannabis derivatives.

YouTube

YouTube videos are not scientific evidence.

Despite what the supporters of these sources may claim, videos and stories are not scientific evidence for the effectiveness of any cancer treatment. Extraordinary claims require extraordinary evidence – YouTube videos are emphatically not scientific evidence, and we are not convinced by them.

Based on the arguments presented on these kinds of websites, it’s impossible to tell whether these patients have been ‘cured’ by cannabis or not. We know nothing about their medical diagnosis, stage of disease or outlook. We don’t know what other cancer treatments they had. We don’t know about the chemical composition of the treatment they got. And we only hear about the success stories – what about the people who have tried cannabis and not been cured? People who make these bold claims for cannabis only pick their best cases, without presenting the full picture.

This highlights the importance of publishing data from scientifically rigorous lab research and clinical trials. Firstly because conducting proper clinical studies enables researchers to prove that a prospective cancer treatment is safe and effective. And secondly because publishing this data allows doctors around the world to judge for themselves and use it for the benefit of their patients.

This is the standard to which all cancer treatments are held, and it’s one that cannabinoids should be held to too. Internet anecdotes and videos prove nothing and benefit no-one – we need reliable, scientific research, which (as discussed above) is exactly what is going on.

“It’s all a big conspiracy – you don’t want people to be cured!”

As we’ve previously said, accusations that we are somehow part of a global conspiracy to suppress cancer cures are as absurd as they are offensive. Not only to the thousands of our scientists, doctors and nurses who are working as hard as they can to find more effective treatments for the complex set of challenging diseases we call cancer, but also the hundreds of thousands of people in the UK and beyond who support this life-saving work through generous donations of money, energy and time.

Race for Life back sign

Our aim is to beat cancer through research

Our aim is to beat cancer, and we believe that the best way to do this through rigorous scientific research aimed at understanding cancer on a biological level and working out how to prevent, detect and treat it more effectively. This approach has helped to change the face of cancer prevention, diagnosis, treatment, leading to a doubling in survival rates over the past 40 years.

As a research-based organisation, we want to see reliable scientific evidence to support claims made about any cancer treatment, be it conventional or alternative.  The claims made for many alternative cancer therapies still require solid evidence to support them, and it often turns out that these ‘miracle cures’ simply don’t work when they’re put to the test.

This doesn’t mean there’s a conspiracy to suppress the “True Cure for Cancer” – it means that doctors and researchers want to see solid evidence that the claims made by people peddling these treatments are true.

This is vital because lives are at stake. Some people may think that a cancer patient has nothing to lose by trying an alternative treatment, but there are big risks.

“What’s the harm? There’s nothing to lose.”

If someone chooses to reject conventional cancer treatment in favour of unproven alternatives, including cannabis, they may miss out on treatment that could save or significantly lengthen their life. They may also miss out on effective symptom relief to control their pain and suffering, or the chance to spend precious time with their loved ones.

Furthermore, many of these unproven therapies come at a high price, and are not covered by the NHS or medical insurance. And, in the worst cases, an alternative therapy may even hasten death.

Although centuries of human experimentation tells us that naturally-occurring cannabinoids are broadly safe, they are not without risks. They can increase the heart rate, which may cause problems for patients with pre-existing or undiagnosed heart conditions. They can also interact with other drugs in the body, including antidepressants and antihistamines. And they may also affect how the body processes certain chemotherapy drugs, which could cause serious side effects.

There is also a reported case where a Dutch lung cancer patient took cannabis extract that had been bought from a street source. Within a matter of hours she was in hospital in a coma. This highlights the risks of taking ‘street’ cannabis extracts of unknown concentration and quality in an uncontrolled way, and accentuates the need for careful research into how best to use cannabinoids for treating patients.

It is a sad fact that although huge progress has been made over recent years, many thousands of people in the UK lose their lives to cancer every year – a situation that we urgently want to change through research. But when conventional treatment fails, there is little chance that turning to an unproven alternative touted on the internet will provide a cure.

In this situation, we recommend that cancer patients talk to their doctor about clinical trials that they may be able to join, giving them access to new drugs and providing valuable data that will help other sufferers in future.

“Big Pharma can’t patent it so they’re not interested.”

Some people argue that the potential of cannabinoids is being ignored by pharmaceutical companies, because they can’t patent the chemicals occurring in cannabis plants. But pharma companies are not stupid, and they are quick to jump on promising avenues for effective therapies.

As we’ve shown, there are hundreds of researchers around the world investigating cannabinoids, in both private and public institutions. And there are many ways that these compounds can be patented – for example, by developing more effective synthetic compounds or better ways to deliver them.

On the flip side, other people argue that patients should be treated with ‘street’ or homegrown cannabis preparations, and that the research being done by companies and other organisations is solely to make money and prevent patients accessing “The Cure”. This is also a false and misleading argument, analogous to suggesting that patients in pain should buy heroin or grow opium poppies rather than being prescribed morphine by a doctor.

The best way to ensure that the benefits of cannabinoids – whether natural or synthetic – are brought to patients is through proper research using quality-controlled, safe, legal, pharmaceutical grade preparations containing known dosages of the drugs.

To do this requires time, effort and money, which may come from companies or independent organisations such as charities or governments. And, ultimately, this investment needs to be paid back by sales of a safe, effective new drug.

We are well aware of the issues around drug pricing and availability – for example, the recent situations with abiraterone and vemurafenib – and we are pushing for companies to make new treatments available at a fair price. We would also hope that if any cannabinoids are shown to be safe and effective enough to make it to the clinic, they would be available at a fair price for all patients that might benefit from them.

“Why don’t you campaign for cannabis to be legalised?”

As things currently stand, cannabis is classified as a class B drug in the UK, meaning that it is illegal to possess or supply it.

It is not for Cancer Research UK to comment on the legal status of cannabis, its use or abuse as a recreational drug, or its medical use in any other diseases. But we are supportive of properly conducted scientific research that could benefit cancer patients.

In summary

At the moment, there simply isn’t enough evidence to prove that cannabinoids – whether natural or synthetic – works to treat cancer in patients, although research is ongoing. And there’s certainly no evidence that ‘street’ cannabis can treat cancer.

As a research-based organisation, we continue to watch the progress of scientists around the world for advances that may benefit people with cancer. And although cannabinoid research is an interesting avenue, it’s certainly not the only one.

Kat

Note: We’ve already entered into two lengthy, time-consuming and ultimately circular debates about cannabis, cannabinoids and cancer which you can read here and here.

Because of this, we are taking the unusual step of keeping public comments closed on this post, as we feel that we have fully laid out our position. If you have a considered comment you would like us to publish on this post you can contact the blog team at scienceblog.cancer.org.uk

Finally, we are grateful to Dr Manuel Guzman (Complutense University, Madrid), Professor Vincenzo di Marzo (Institute of Biomolecular Chemistry, Naples, and GW Pharmaceuticals) and Dr Wai Liu (St George’s Hospital, London) for helpful discussions as we were writing this post.


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Kat Arney July 4, 2014

Additional information:

We often see websites with long lists of scientific papers claiming that cannabis is a “cure” for various cancers. However, when we look at the detail of the data and the experimental detail of the research, it becomes clear that although they may be interesting and build evidence to show that cannabinoids may one day bring benefits for cancer patients, they are far from being a cure.

The main point to realise is that virtually all these studies have been done in cancer cells grown in the lab or in animals. These are quite artificial systems and are much less complex than a real cancer growing in a patient.

For example, most experiments with cells grown in the lab use cancer cells that were originally taken from a tumour many years ago, but have been grown for a long time in the lab – known as cell lines. One problem with such cells is that they are all very similar on a genetic and molecular level, but we know that in real cancers, the cells can be very different from each other and respond in different ways to treatments. Also the usual way of testing cannabinoids in animals has been done by transplanting cancer cells (either mouse or human) into mice. Usually only a small number (5-20) will be used for each experiment.

There’s growing evidence that these particular kinds of models (known as xenografts) aren’t as good at suggesting a treatment could work, compared to more sophisticated genetically engineered animals, as they don’t accurately represent the situation in real tumours. So although these kinds of experiments can point towards useful approaches, as well as revealing the underlying molecular ‘nuts and bolts’ of what’s going on, they can’t tell us if something will definitely treat cancer effectively and safely in human patients. They do not “prove that cannabis cures cancer”, as the headlines would have us believe.

Put simply, Petri dishes are not people. Most chemicals that show promise in lab or animal experiments turn out not to work as well as hoped when tested in patients. These kinds of human studies, known as clinical trials, are the only way we can really know if a cancer treatment is effective. There’s more about clinical trials on our website: http://www.cancerresearchuk.org/cancer-help/trials/types-of-trials/

It’s also important to think about what’s being claimed when people use the word “cure”. To most people, including us, this means that a cancer is completely treated and does not come back. When we look at the data in the papers listed below, none of them come close to showing these kinds of results. For the experiments involving cells grown in the lab, a proportion of the cells are killed or stop growing, but some of them carry on. Similarly in animal experiments, there is no data that shows a 100 per cent success rate for cannabinoids. For example, most mice treated with cannabinoids will still have tumours, although the cancers may be growing more slowly and spread less in some of them.

This isn’t just true for cannabinoids – it’s true for virtually all cancer drugs used today. Cancer is a very complex biological problem – there are hundreds of different types of cancer, each with important molecular and genetic differences. There’s good evidence to show that every individual’s cancer is as unique as they are, and that tumours can evolve and change within the body to become resistant to treatments.

We know that cancer drugs don’t work for everyone all the time – that’s why there’s so much effort going on to find more effective treatments – but it’s vital that doctors have a solid body of evidence showing how well the treatments they’re using are likely to work. If you or someone you loved were going to take any kind of drug, would you be happy if it had only been tested in very high doses on cancer cell lines grown in the lab, or in mice injected with cancer cells? Or would you want to know that it had been trialled in large numbers of people, and there was good data on how effective it is, whether it’s safe in the dose given, what the side effects are, and the proportion of people that can be expected to get better?

This kind of evidence can only come from a combination of lab studies leading to clinical trials. At the moment, while there are hundreds of interesting lab studies of cannabinoids (just some of which are included in the list below) there is only one clinical trial that has been published. So for now, cannabinoids, whether natural or synthetic, are a very long way from being what we would describe as a “cure” for any type of cancer.

We’ve looked at each of the papers in one of the commonly-seen lists (for example, here), and noted down the kinds of experiments they are. Many of them are available as open access papers, so it’s possible to look at the data for yourself. Hopefully this is a useful explanation of the kind of scientific research that is currently ongoing into cannabinoids and cancer, and the process of gathering evidence to show whether a potential cancer therapy works.

Brain Cancer
http://www.nature.com/bjc/journal/v95/n2/abs/6603236a.html These are the only published results of any clinical trial we are aware of testing cannabis for cancer, which we cover in the blog post. Nine people with advanced, terminal glioblastoma multiforme – an aggressive brain tumour – were given highly purified tetrahydrocannabinol (THC) through a tube directly into their brain. Eight people’s cancers showed some kind of response to the treatment, and one didn’t respond at all. All the patients died within a year, as might be expected for people with cancer this advanced.

http://www.ncbi.nlm.nih.gov/pubmed/11479216 This is a study of a synthetic cannabinoid tested in mice that have had rat brain tumour cells from a cell line injected into them.

http://www.jneurosci.org/content/21/17/6475.abstract This paper isn’t about cancer.

http://jpet.aspetjournals.org/content/308/3/838.abstract This is a study using human brain tumour cell lines grown in the lab.

http://mct.aacrjournals.org/content/10/1/90.abstract This is a study testing the combined effects of temozolomide, a drug used to treat brain tumours, with cannabinoids in brain tumour cells grown in the lab and also mice that have been transplanted with a human brain tumour cell line.

Mouth and Throat Cancer
http://www.ncbi.nlm.nih.gov/pubmed/20516734 This is a study testing the effects of cannabinoids on a mouth cancer cell line growing in the lab.

Breast Cancer
http://www.ncbi.nlm.nih.gov/pubmed/20859676 This is a study with human breast cancer cell lines grown in the lab and mice that have been transplanted with mouse mammary (breast) cancer cells from a cell line.

http://www.ncbi.nlm.nih.gov/pubmed/18025276 This is a study investigating human breast cancer cell lines grown in the lab.

http://www.ncbi.nlm.nih.gov/pubmed/21915267 This is a study looking at whether samples of human breast tumours carry the receptors for cannabinoids. The researchers also test the effects of a synthetic cannabinoid on human breast cancer cells lines grown in the lab, and in mice that have been transplanted with a breast cancer cell line.

http://jpet.aspetjournals.org/content/early/2006/05/25/jpet.106.105247.full.pdf+html This papers looks at the effects of cannabinoids on human breast cancer and leukaemia cell lines grown in the lab, and also on mice that have been transplanted with rat thyroid cells that have been infected with a virus to make them cancerous, or cells from a human breast cancer cell line.

http://www.molecular-cancer.com/content/9/1/196 This study investigates whether samples of human breast cancers carry cannabinoid receptors. The researchers also test cannabinoids (THC and a synthetic cannabinoid) in genetically modified mice that are prone to developing breast cancers.

http://www.ncbi.nlm.nih.gov/pubmed/22776349 This is a review of the research into cannabinoids and cancer that had been carried out up until 2012. It is not a research paper.

http://www.pnas.org/content/95/14/8375.full.pdf+html This paper looks at the effects of an endocannabinoid (anandamide, produced by the body) on human breast cancer cell lines grown in the lab.

Lung Cancer
http://www.ncbi.nlm.nih.gov/pubmed/22198381?dopt=Abstract This paper looks at the effects of cannabidiol (CBD) on human lung cancer cell lines, and also mice that have been transplanted with cells from a lung cancer line.

http://www.ncbi.nlm.nih.gov/pubmed/21097714?dopt=Abstract This paper looks at the effects of synthetic cannabinoids on human lung cancer cell lines grown in the lab, and on mice that have been transplanted with a human lung cancer cell line.

http://www.nature.com/onc/journal/v27/n3/abs/1210641a.html This paper looks at the effects of THC on human lung cancer cell lines grown in the lab, and on mice that have been transplanted with a human lung cancer cell line.

Uterine, Testicular, and Pancreatic Cancers
http://www.cancer.gov/cancertopics/pdq/cam/cannabis/healthprofessional/page4 This is an overview of some of the animal and cell experiments that have been done. It is not a research paper.

http://cancerres.aacrjournals.org/content/66/13/6748.abstract This study looked at the effects of THC and a synthetic cannabinoid on two pancreatic cancer cells lines grown in the lab, and also in mice that had been transplanted with cells from a pancreatic cancer line. The researchers also show that human pancreatic cancer samples contain cannabinoid receptors.

Prostate Cancer
http://www.ncbi.nlm.nih.gov/pubmed/12746841?dopt=Abstract This paper looks at the effects of anandamide on three different human prostate cancer cell lines.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3339795/?tool=pubmed This is a review of research about cannabinoids and prostate cancer. It is not a research paper.

http://www.ncbi.nlm.nih.gov/pubmed/22594963 This looks at the effects of non-THC cannabinoids (CBD and related compounds) on different human prostate cancer cell lines, and also on mice that have been transplanted with cells from two human prostate cancer lines.

Colorectal Cancer
http://www.ncbi.nlm.nih.gov/pubmed/22231745 This looks at whether CBD can reduce the incidence of bowel cancer in mice that have been treated with a chemical that causes tumours. The researchers also looked at bowel cancer cell lines grown in the lab.

Ovarian Cancer
http://www.aacrmeetingabstracts.org/cgi/content/abstract/2006/1/1084 This is an abstract from research that was presented at a meeting, so it’s not possible to see the data or methods. From the abstract, it looks like the researchers have investigated the levels of cannabinoid receptors in human ovarian cancer cell lines grown in the lab, and also looked at the effects of a synthetic cannabinoid on these cell lines.

Blood Cancer
http://www.ncbi.nlm.nih.gov/pubmed/12091357 This paper looks at a range of mouse and human leukaemia and lymphoma cell lines grown in the lab, looking for the presence of cannabinoid receptors and their response to THC or a synthetic cannabinoid. They also test the effects of THC on mice that have been transplanted with mouse lymphoma cells. Two out of eight mice were still alive at the end of the experiment.

http://www.ncbi.nlm.nih.gov/pubmed/16908594 This paper looks at the effects of THC on a human leukaemia cell line grown in the lab, aiming to understand how the drug might be affecting the cells.

http://onlinelibrary.wiley.com/doi/10.1002/ijc.23584/abstract In this paper the researchers look at whether samples of Hodgkin’s lymphoma from patients carry cannabinoid receptors. They also test the effects of a chemical based on anandamide in a human leukaemia cell line.

http://molpharm.aspetjournals.org/content/70/5/1612.abstract This looks at the effects of an anandamide-like chemical and synthetic cannabinoids on lymphoma cell lines and samples of lymphoma from patients. It’s focusing on the molecular ‘nuts and bolts’ of what’s going on in the cells rather than testing how well they work.

Skin Cancer
http://www.ncbi.nlm.nih.gov/pubmed/12511587 This paper looks at the effects of synthetic cannabinoids on mouse and human skin cancer cell lines grown in the lab, as well as mice that had been injected with mouse skin cancer cells – these were non-melanoma skin cancer cell lines, rather than melanoma. They also looked at the pattern of cannabinoid receptors in samples of human skin cancers.

Liver Cancer
http://www.ncbi.nlm.nih.gov/pubmed/21475304 This paper looks at the effects of THC and a synthetic cannabinoid on human liver cancer cell lines grown in the lab. They also looked at the effects of these drugs on mice that had been transplanted with cells from a human liver cancer line.

Biliary Tract Cancer
http://www.ncbi.nlm.nih.gov/pubmed/19916793 The link to this paper isn’t working so we’re unable to see the data or methods. From the abstract, it appears the researchers have tested THC on cholangiocarcinoma (bile duct cancer) cells grown in the lab, and also looked at patterns of cannabinoid receptors in samples of human bile duct tumours.

Bladder Cancer
http://www.medscape.com/viewarticle/803983 This is a report from a scientific meeting looking at whether smoking cannabis affects bladder cancer risk. The data have not been published in a scientific journal so it isn’t possible to look at them in detail or draw any firm conclusions.

Cancer in General
http://www.ncbi.nlm.nih.gov/pubmed/12514108 This paper looks at the effects of cannabinoids on blood vessel growth in tumours. The researchers test a synthetic cannabinoid on mice that have been transplanted with brain tumour cells, looking in particular at the tumour blood vessels, and also look at the effects of cannabinoids on human blood vessel cells growing in the lab.

http://www.ncbi.nlm.nih.gov/pubmed/15313899 This paper looks at how activity levels of certain genes in mouse brain tumours change when given a synthetic cannabinoid, focusing on genes involved in the growth of new blood vessels. The researchers also look at the effects of cannabinoids on human brain tumour cells and other cancer cells growing in the lab, and at certain molecular markers in tumour samples taken from patients involved in the clinical trial described in the first paper in this list (Guzman et al http://www.nature.com/bjc/journal/v95/n2/abs/6603236a.html )