Due to the pandemic, there have been delays in updating this article as new research emerges. The last update was May 2022, to reflect the latest research and ongoing clinical trials.

For the last couple of decades, one of the most talked about discussions online is whether or not cannabis can treat cancer.

Claims that there is solid “proof” that cannabis or cannabinoids can cure cancer are highly misleading. 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.

The basics

What is cannabis?

Cannabis is a plant grown and cultivated commercially across the globe. It is known by many names depending on its preparation and quality, including marijuana, trees, pot, dank, grass, green, kush, weed, hemp, hash, loud, and herb. These usually refer to the dried form or resin of the flowers or leaves of the plant.

There are multiple species of cannabis plant, including Cannabis sativa, Cannabis indica and Cannabis ruderalis.

For thousands of years, it has been used recreationally, religiously, and medically. Records from Ancient Egypt, India, and China show that physicians would use the plant as part of treating ailments such as haemorrhoids, insomnia, and for other pain relief.

In the Western world, cannabis emerged as a mainstream medicine in the 1840s and was noted for its sedative, anti-inflammatory, pain relief, and anticonvulsant effects.

Scientists have identified multiple active compounds within cannabis (known as cannabinoids) that play a role in cannabis’ effects, including the psychoactive delta-9-tetrahydrocannabinol (THC) and cannabidiol (CBD).

Cannabinoids – what are they?

Cannabinoids are compounds that can interact with a system inside the body known as the endocannabinoid system.

Most commonly, the term “cannabinoid” is used to refer to the compounds found in cannabis (and other plants). As the body naturally produces cannabinoids itself (known as endocannabinoids), a more accurate term for these is phytocannabinoids (meaning “cannabinoids from plants”).

Researchers have found that cannabis contains over 450 different chemical compounds, many of which are cannabinoids.

The two main cannabinoids of interest to researchers 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.

Is cannabis legal in the UK?

In the UK, medical use of cannabis was legalised in November 2018 and the UK is one of the world’s largest exporters of legal cannabis. However, cannabis is still classified as a class B drug in the UK, meaning that it is illegal to possess or supply it for personal recreational use.

Medical cannabis is only legal when prescribed by a specialist consultant and GPs are not allowed to prescribe cannabis-derived medicines. NHS guidance states that medical cannabis should only be prescribed when there is clear published evidence of its benefit and other treatment options have been exhausted.

The National Institute for Health and Care Excellence (NICE) has listed conditions where it believes medically prescribed cannabis products might be helpful.

How do cannabinoids work inside the body?

Our bodies naturally produce our own cannabinoids (known as endocannabinoids).

These interact with molecules found on the surface of cells (cannabinoid receptors). One type of is densely packed inside the brain and second type is found in our immune tissues.

These compounds and receptors form the endocannabinoid system, a network that is involved in the control and regulation of multiple functions within the body – including memory, sleep, learning, eating, pain control, inflammation, and immune system.

As THC, CBD and other cannabinoids look similar to the endocannabinoids inside the body, they are able to interact with these receptors and affect how the system functions.

This is why some researchers think that cannabinoids have the potential to control some of the most common and debilitating symptoms of cancer and its treatments, including nausea and vomiting, loss of appetite, and pain.

Is all cannabis is the same?

Like how beer, wine, and vodka all have differing levels of alcohol and other ingredients, different strains/types of cannabis have varying levels of THC, CBD, and other compounds. This means that different strains of cannabis can have different effects on the body.

Additionally, its effects also depend on how cannabis is taken, most commonly by inhaling (smoking or vaping) or ingesting (edibles).

When it is inhaled, THC enters the lungs where it passes directly into your bloodstream and then your brain quickly. The effects of inhaled cannabis fade faster than cannabis taken by mouth.

When ingested (such as when it’s used in oils/drinks/baked goods/sweets), edible cannabis travels first to your stomach then to your liver before getting into your bloodstream and brain. The liver converts THC into a stronger compound and this (combined with the THC from the original product) adds to the intensity of the high.

Are there cannabis-based medicines?

Some cannabis-based products are available on prescription. The following medicines are sometimes prescribed to help relieve symptoms.

Nabilone

Nabilone is a drug developed from cannabis. It is licensed for treating severe sickness from chemotherapy that is not controlled by other anti-sickness drugs.

It works very well for some people, but can cause drowsiness or dizziness in others. These side effects can last for a couple of days after you’ve stopped taking it.

Sativex

Sativex (or nabixmols) is a liquid cannabis-based medicine that you spray into your mouth.

Researchers are looking into Sativex as a treatment for cancer related symptoms and for certain types of cancer.

The research

Is Cancer Research UK investigating cannabinoids?

In the past, 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 have also supported the work of 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.

How do researchers study cannabis?

Around the world, many researchers are actively investigating cannabis and cannabinoids, and Cancer Research UK is supporting some of this work.

Generally, the cannabis that researchers study isn’t the same as the one as you might see on the street or oils sold in shops.

When researchers conduct rigorous scientific studies, they often use purified forms of the compounds that they are investigating . This gives us more reliable evidence on the effect of different cannabinoids.

Through many detailed experiments – summarised in this review article from the British Journal of Cancer – scientists have discovered that both natural and synthetic cannabinoids have a wide range of effects on cells, which is why there’s interest in finding out whether it can be a part of treating diseases like cancer, as well as help relieving side effects.

Can cannabinoids treat cancer?

As of 2022, several hundreds of scientific papers looking at cannabinoids, the endocannabinoid system, and the relation to cancer have been published. So far these studies simply haven’t found enough robust scientific evidence to prove that these can safely and effectively treat cancer.

This is because the majority of 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 CBD . 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, CBD, and other cannabinoids could treat people with glioblastoma multiforme brain tumours that have come back after treatment.

In 2021, scientists reported the final results of this phase 1 study to treat people with recurrent glioblastoma with Sativex in combination with the chemotherapy drug, temozolomide. Researchers found that adding Sativex (patients were allowed to choose the amount they took) had acceptable levels of side effects, which included vomiting, dizziness, fatigue, nausea and headache. They also observed that more patients were alive after one year using Sativex (83%) compared to those taking the placebo (44%). However, this phase 1 study only involved 27 patients, which was too small to confirm any potential benefits of Sativex, and was intended to find out if it was safe to take by patients.

This trial is being extended into phase 2 (known as ARISTOCRAT) to explore if this treatment is effective and which patients are most likely to respond to this treatment. It is set to launch at 15 NHS hospitals in 2022, with over 230 patients to be recruited (and making use of the Cancer Research UK Clinical Trials Unit). To find out more about this work, you can listen to our podcast – That Cancer Conversation – where we hear from Professor Susan Short, one of the researchers leading this study.

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.

Can cannabis prevent cancer?

There is no reliable evidence that cannabis can prevent cancer.

There has been some research suggesting that the body’s endocannabinoids (mentioned earlier) can suppress tumour growth.

When it comes to cannabis, experiments where mice were given very high doses of purified THC showed that 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 cannabis cause cancer?

The evidence is a lot less clear when it comes to whether cannabis itself can cause cancer.

This is because most people who use cannabis smoke it mixed with tobacco, a substance that we know causes cancer. Data from 2016 has shown that 77% of UK people surveyed (who smoke weed) reported normally mixing it with tobacco.

This makes it hard for researchers to disentangle the potential impact of cannabis on cancer risk from the impact of the tobacco. As of 2022, we can’t be sure whether the increased risk is due to tobacco or whether cannabis itself 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.

In addition to this, there have not been published studies looking at cannabis ingestion (such as edibles) and cancer risk, nor vaporised cannabis and cancer risk.

Can cannabis relieve cancer symptoms like pain or sickness?

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.

Questions that still need to be answered

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.

Frequently asked questions (FAQs)

“What’s Cancer Research UK’s view on cannabis and cancer?”

As of 2022, Cannabis is still classified as a class B drug in the UK, meaning that it is illegal to possess or supply it for personal recreational use.

Cancer Research UK does not have an organisational policy 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 and we will continue to monitor developments in the fields and evidence as it emerges.

“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, 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 was a case report published in the British Medical Journal describing a woman in her 80s with lung cancer whose tumour shrank after taking CBD oil over several months.

This might seem like a solid bit of proof, 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

The authors state that even though this case appears to demonstrate a possible benefit of CBD oil intake, it’s not possible to confirm that the tumour regression was due to the patient taking CBD oil (as she was also taking drugs for other conditions).

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 cannabis and cannabinoids should be held to, too.

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

If someone chooses to complete reject conventional cancer treatment in favour of unproven alternatives, 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 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.

As cannabis is illegal for recreational use in the UK, there are further risks associated with using home-made preparations, particularly cannabis oil, such as toxic chemicals left from the solvents used in the preparation process.

Synthetic cannabinoids (sometimes known as spice) are compounds that have been designed to act like the chemicals found in cannabis but with far stronger effects and have harmful side-effects associated.

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.

“Are cancer charities hiding cannabis as a cure?”

The idea that a cure already exists is one of the many myths that surrounds cancer that we have written about.

This myth is unjust 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 increased survival in the last few decades.

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.

“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 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 homegrown cannabis preparations, and that the research being done by companies is solely to make money and prevent patients accessing “the cure”

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.

In summary

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 cannabis bought on the street 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.