We review and update this article as new research emerges. The last update was April 2021, when we updated the section on questions around whether cannabis can causes or prevent cancer.
Few cancer topics spark as much online debate as cannabis.
The bottom line is that right now there isn’t enough reliable evidence to prove that any form of cannabis can effectively treat cancer in patients. This includes hemp oil, cannabis oil or the active chemicals found within the cannabis plant (cannabinoids) – whether natural or man-made.
Many researchers worldwide are actively investigating cannabinoids, and Cancer Research UK is supporting some of this work. These studies use highly purified chemicals found in the cannabis plant, or lab-made versions of them, and there is genuine interest in these as potential cancer treatments. But this is very different to street-bought cannabis and hemp oil available online or on the high street, for which there is no evidence of any impact on cancer.
Cannabis is still classified as a class B drug in the UK, meaning that it is illegal to possess or supply it. Cancer Research UK can’t comment on the legal status of cannabis, its use as a recreational drug, or its medical use in any other diseases. But we are supportive of properly conducted scientific research into cannabis and its derivatives that could benefit cancer patients.
Unfortunately, there are many unreliable sources of information about cannabis, particularly online. This post contains up-to-date, evidence-based information on cannabis and cancer, so with lots to cover, this is a long article. To help you find what you’re interested in, follow the links below to different sections. Or read on for everything you need to know about cannabis and cancer.
- Cannabis and cannabinoids – what are they?
- Can cannabinoids treat cancer?
- Can cannabis prevent or cause cancer?
- What about controlling cancer symptoms such as pain or sickness?
- Is Cancer Research UK investigating cannabinoids?
- It’s natural so it must be better, right?
- “Are cancer charities hiding cannabis as a cure?”
- “What’s the harm? There’s nothing to lose.”
- “Big Pharma can’t patent it so they’re not interested.”
- “Why doesn’t Cancer Research UK campaign for cannabis to be legalised?”
Cannabis and cannabinoids – what are they?
Cannabis is a plant known by many names, including marijuana, pot, grass, weed, hemp, hash or dope.
The plant produces a resin that contains complex chemicals called cannabinoids.
The two main cannabinoids are:
- Delta-9-tetrahydrocannabinol (THC) – a psychoactive substance that can affect how the brain works, creating a ‘high’ feeling.
- Cannabidiol (CBD) – may relieve pain, lower inflammation and decrease anxiety without any psychoactive effects.
Cannabinoids lock on to molecules on the surface of cells called cannabinoid receptors. As well as cannaibinoids found in plant resin, our body produces cannabinoid chemicals – called endocannibinoids – which also attached to these receptors. These receptors are involved in many processes throughout the body, from appetite to the sensation of pain.
Through many detailed experiments – summarised in this Nature Reviews Cancer article – scientists have discovered that both natural and synthetic cannabinoids have a wide range of effects on cells, which is why there’s interest in if cannabis can treat diseases like cancer, as well as help relieve side effects.
Can cannabinoids treat cancer?
Many hundreds of scientific papers looking at cannabinoids and cancer have been published so far, but these studies simply haven’t found enough robust scientific evidence to prove that these can safely and effectively treat cancer.
Research is still ongoing though, with hundreds of scientists investigating the potential of cannabinoids in cancer and other diseases as part of The International Cannabinoid Research Society. And in 2015 the scientific journal Nature published a supplement of review articles about various aspects of cannabis. It’s free to access.
Claims that there is solid “proof” that cannabis or cannabinoids can cure cancer is highly misleading.
This is because virtually all the scientific research investigating whether cannabinoids can treat cancer has been done using cancer cells grown in the lab or animals. While these studies are a vital part of research, providing early indications of the benefits of particular treatments, they don’t necessarily hold true for people.
So far, the best results from lab studies 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 man-made cannabinoids, such as a molecule called JWH-133.
There have been intriguing results from lab experiments looking at a number of different cancers, including glioblastoma brain tumours, prostate, breast, lung, and pancreatic cancers. 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.
There’s also evidence that cannabinoids have unwanted effects. Although high doses of THC can kill cancer cells, they also harm crucial blood vessel cells. And under some circumstances, cannabinoids can encourage cancer cells to grow, or have different effects depending on the dose used and levels of cannabinoid receptors present on the cancer cells.
Cannabis in clinical trials
To robustly test the potential benefits of cannabinoids in cancer, clinical trials in large numbers of people with control groups of patients – who aren’t given the treatment in question – would be needed.
A few small clinical trials have been set up to test the benefits of cannabinoids for people with glioblastoma multiforme. Results published from a pilot clinical trial where 9 people with advanced, incurable glioblastoma multiforme – the most aggressive brain tumour – were given highly purified THC through a tube directly into their brain showed that THC given in this way is safe and doesn’t seem to cause significant side effects. But as this was an early stage trial without a control group, it couldn’t show whether THC helped to extend patients’ lives.
And a second clinical trial, supported through our Experimental Cancer Medicine Centre (ECMC) Network, tested whether Sativex (nabiximols), a highly purified pharmaceutical-grade extract of cannabis containing THC and CBD, could treat people with glioblastoma multiforme brain tumours that have come back after treatment. Initial results from this trial, which came out in 2018, showed that combining Sativex with the chemotherapy temozolomide increased the number of patients who were alive one year after their brain tumour came back from 44% to 83%.
While these are promising early stage findings, this was a small trial with 21 patients taking part, so larger clinical trials are needed to confirm what the benefits of Sativex might be, and which patients are most likely to respond to this combination of treatments.
We’ve also supported a trial that’s testing the benefits of a man-made cannabinoid called dexanabinol in patients with different types of advanced cancer. The trial finished recruiting in 2015 and researchers established a safe dose of the drug, but further development of the drug was stopped due to a lack of evidence around the drug’s effectiveness. Full trials results are yet to be published.
There are still many unanswered questions around the potential for using cannabinoids to treat cancer. It’s not clear:
- which type of cannabinoid – either natural or synthetic – might be most effective
- what kind of doses might be needed
- which types of cancer might respond best to cannabinoids
- how to avoid the psychoactive effects of THC
- how best to get cannabinoids, which don’t dissolve easily in water, into cancer cells
- whether cannabinoids will help to boost or counteract the effects of chemotherapy
These questions must be answered for cannabinoids to be used as safe and effective treatments for cancer patients. It’s the same situation for the many hundreds of other potential cancer drugs being developed and tested in university, charity and industry labs all over the world.
Without doing rigorous scientific research, we will never sift the ‘hits’ from the ‘misses’. If cannabinoids are to get into the clinic, these hurdles first need to be overcome and their benefits proven over existing cancer treatments.
There is no reliable evidence that cannabis can prevent cancer.
There has been some research suggesting that endocannabinoids (mentioned earlier) can suppress tumour growth, and in experiments where mice were given very high doses of purified THC, they seemed to have a lower risk of developing cancer. But this is not enough solid scientific evidence to suggest that cannabinoids or cannabis can cut people’s cancer risk.
Does smoking weed cause cancer?
The evidence is a lot less clear when it comes to whether cannabis can cause cancer.
This is because most people who use cannabis smoke it mixed with tobacco, a substance that we know causes cancer. In the UK, more than three quarters of people who smoke weed reported normally mixing it with tobacco.
This makes it hard to disentangle the potential impact of cannabis on cancer risk from the impact of the tobacco. As of 2021, we can’t be sure whether the increased risk is due to tobacco or whether cannabis also has an independent effect.
We do know from decades of evidence that there is no safe way to use tobacco – it’s addictive and harmful for your health. People who smoke weed mixed with tobacco increase their risk of cancer and other conditions. Tobacco also contains the very addictive substance nicotine. This means people who regularly smoke weed mixed with tobacco may find it harder to stop.
> Read about the free support and quitting tools available to help you to stop smoking for good on our website.
There’s good evidence that cannabinoids may be beneficial in managing cancer pain and side effects from treatment.
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, medical marijuana has been legalised for relieving pain and symptoms (palliative use), including cancer pain. But one of the problems with using herbal cannabis is managing the dose. Smoking cannabis or taking it in the form of tea often provides an inconsistent 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 in the UK have been testing whether a mouth spray formulation of Sativex (nabiximols) can help to control severe cancer pain that doesn’t respond to other drugs. Results from these didn’t find any difference in self-reported pain scores between the treatment and the placebo.
Cannabinoids may also have potential in combating the loss of appetite and wasting (cachexia) experienced by some people with cancer, although so far clinical evidence is lacking. One clinical trial comparing appetite in groups of cancer patients given cannabis extract, THC and a placebo didn’t find a difference between the treatments, while another didn’t show any benefit and was closed early.
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 a review looking at the potential of cannabinoids for treating bowel cancer.
We also support Dr Laureano de la Vega, a Cancer Research UK Fellow at the University of Dundee, who in 2019 started to explore if CBD can limit cancer’s ability to spread, using lung and triple negative breast cancer cells grown in the lab.
We’re also involved in the only 2 UK clinical trials of cannabinoids for treating cancer, mentioned above, through our national network of Experimental Cancer Medicine Centres.
Our funding committees have previously received other applications from researchers who want to investigate cannabinoids but these failed to reach our high standards for funding. If we receive future proposals that meet these stringent requirements, then there is no reason that they wouldn’t be funded, assuming we have the money available.
Unfortunately, some scammers are using the email address firstname.lastname@example.org and claiming to be based at our head office, tricking cancer patients and their families into handing over money for “cannabis oil”, after which they receive nothing in return. This is a scam and has nothing to do with Cancer Research UK or our employees, as we wrote about in 2015. If you believe you have been a victim of this fraud, please contact the police.
There’s no doubt that the natural world is a treasure trove of biologically useful compounds, and there are countless examples where these have been harnessed as effective treatments.
Numerous potent cancer drugs have also been developed in this way – purifying a natural compound, improving it and testing it to create a beneficial drug – including taxol, vincristine, vinblastine, camptothecin, colchicine, and etoposide. But although these purified drugs in controlled high doses can treat cancer, it 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.
“But it worked for this patient…”
Doctors sometimes publish case reports about extraordinary or important observations they have seen in their clinic. For example, there is 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 very little reliable information can be taken from a single patient treated with what’s an unknown mix of cannabinoids outside of a controlled clinical setting.
There are also many videos and anecdotes online claiming that people have been completely cured of cancer with cannabis, hemp/cannabis oil or other cannabis derivatives.
Despite what these sources may claim, it’s impossible to tell whether these patients have been ‘cured’ by cannabis or not. There is usually no information about their medical diagnosis, stage of disease, what other cancer treatments they had, or the chemical make-up of their treatment. These sources also only publish the “success stories”, and don’t share how many people who used cannabis or its derivatives had no benefit, or worse, were potentially harmed.
Robust scientific studies describe the detail of experiments and share the results – positive or negative. This is vital for working out whether a potential cancer treatment is truly safe and effective, or not. And publishing this data allows doctors around the world to judge the information for themselves and use it for the benefit and safety 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.
Dr Wai Liu at St George’s University is researching cannabis and cannabinoids for treating cancer to build up the evidence. 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.
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 pain or other problems.
Many of these unproven therapies are also expensive, and aren’t covered by the NHS or medical insurance. In the worst cases, an alternative therapy may even hasten death.
Although centuries of human experimentation tell us that naturally-occurring cannabinoids are broadly safe, they are not without risks. They can increase 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.
Cannabis is an illegal (class B) drug in the UK and there are further risks associated with using black market or home-made preparations, particularly cannabis oil, such as toxic chemicals left from the solvents used in the preparation process.
There are also many internet scams by people offering to sell cannabis preparations. As well as the risk of getting something with completely unknown chemical or medicinal properties and unknown effectiveness, scammers are tricking cancer patients and their families into handing over money for “cannabis oil” which they then never receive.
We understand the desire to try every possible avenue when conventional cancer treatment fails. But there is little chance that an unproven alternative treatment bought online will help, and it may well harm. 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 in a safe and monitored environment.
We’ve blogged previously about how unjust this is to the thousands of scientists, doctors and nurses working as hard as they can to beat cancer, and to the many thousands of people in the UK and beyond who give up their time and money to fund our work.
History shows that the best way to beat cancer is through rigorous scientific research. This approach has helped to change the face of cancer prevention, diagnosis, treatment, leading to survival doubling 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. 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.
Some people argue that the potential of cannabinoids is being ignored by pharmaceutical companies, because they can’t patent the chemicals naturally occurring in cannabis plants. But there are many ways that these compounds can be patented – for example, by developing more effective lab-made versions or better ways to deliver them.
Other people argue that patients should be treated with ‘street’ or homegrown cannabis preparations, and that the research being done by companies is solely to make money and prevent patients accessing “the cure”.
But the best chance of ensuring that the potential benefits of cannabinoids – whether natural or man-made – can be brought to patients is through research using quality-controlled, safe, legal, pharmaceutical grade preparations containing known amounts of the drugs.
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.
It’s true that there are issues around drug pricing and availability and we’re pushing for companies to make new treatments available at a fair price. We would hope that if cannabinoids were to be shown to be safe and effective enough to make it to the clinic, they would be made available at a fair price for all patients who might benefit from them.
Cannabis is classified as a class B drug in the UK, meaning that it is illegal to possess or supply it.
Cancer Research UK does not have an organisational policy 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 into cannabis and its derivatives that could benefit cancer patients and we will continue to monitor developments in the fields and evidence as it emerges.
Right now, there simply isn’t enough evidence to prove that cannabinoids – whether natural or synthetic – can effectively treat cancer in patients, although research is ongoing. And there’s certainly no evidence that ‘street’ cannabis can treat cancer.
We’re supportive of properly conducted scientific research into cannabis and its derivatives that could benefit cancer patients. Many researchers are actively exploring this approach, and Cancer Research UK is supporting, and will continue to support, scientifically robust research into cannabis and cannabinoids that reaches the high-quality standards set by our funding committees.
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References and further reading:
- CancerHelp UK – Does smoking cannabis cause cancer?
- CancerHelp UK – Is cannabis a treatment for brain tumours?
- CancerHelp UK – Two trials of Sativex for cancer-related pain
- National Cancer Institute (US) – Information about cannabis and cannabinoids for cancer patients
- National Cancer Institute (US) – Information about cannabis and cannabinoids for health professionals
- Velasco, G., Sánchez, C. & Guzmán, M. (2012). Towards the use of cannabinoids as antitumour agents, Nature Reviews Cancer, 12 (6) 444. DOI: 10.1038/nrc3247
- Sarfaraz, S. et al (2008). Cannabinoids for Cancer Treatment: Progress and Promise, Cancer Research, 68 (2) 342. DOI: 10.1158/0008-5472.CAN-07-2785
- Guindon, J. & Hohmann, A.G. (2011). The endocannabinoid system and cancer: therapeutic implication, British Journal of Pharmacology, 163 (7) 1463. DOI: 10.1111/j.1476-5381.2011.01327.x
- Engels, F.K. et al (2007). Medicinal cannabis in oncology, European Journal of Cancer, 43 (18) 2644. DOI: 10.1016/j.ejca.2007.09.010
- Cannabinoids in the treatment of chemotherapy-induced nausea and vomiting – Todaro (2012) Journal of the National Comprehensive Cancer Network
- Bowles, D.W. et al (2012). The intersection between cannabis and cancer in the United States, Critical Reviews in Oncology/Hematology, 83 (1) 10. DOI: 10.1016/j.critrevonc.2011.09.008
- Hall, W., Christie, M. & Currow, D. (2005). Cannabinoids and cancer: causation, remediation, and palliation, The Lancet Oncology, 6 (1) 42. DOI: 10.1016/S1470-2045(04)01711-5.
- Why anti-cancer properties in cannabis must be investigated, Wai Liu, The Conversation