Cannabis oil and cancer

Many people want to know more about cannabis and cancer. It is important to find reliable information.

What is cannabis?

You have probably heard many names for cannabis, including marijuana, hemp, pot, grass and hash. Cannabis has been used both medicinally and recreationally for thousands of years.

The cannabis plant produces a resin that contains different substances. Some of these may have medicinal value. These substances are called cannabinoids.

Two of the main cannabinoids are:

  • delta-9-tetrahydrocannabidiol (THC) – it is illegal to use this in the UK
  • cannabidiol (CBD) – this is legal in the UK.

There are some medicines available in the UK that come from cannabis. These include Sativex®, which is used to treat multiple sclerosis, and Nabilone®, which is used to relieve sickness caused by chemotherapy.

These medicines go through the same process as any other medicine to make sure they are safe to be licensed and prescribed in the UK.

Cannabis and cancer

There is a lot of interest in the possible effects of cannabinoids on cancer. Some people claim that using CBD has had a positive effect on their health. But these are individual stories, not scientific evidence. There is no way of checking whether the stories are reliable, or finding out what other treatments the person has had.

Scientists have researched THC and CBD to see if they could have any role in the treatment of cancer. Most of this research is at a very early stage and is still being done in the laboratory. It has not been tested in humans. This means there is no reliable, medical evidence to show whether cannabis, in any form, can effectively and safely treat cancer in humans.

What does the research say?

Research has found that cannabinoids can:

  • cause a cell to die
  • stop cells from dividing
  • stop cells from developing new blood vessels.

But it has also found that cannabinoids can:

  • damage important blood vessels
  • in some situations, encourage cancer cells to grow
  • make the immune system less effective – the immune system fights diseases such as cancer.

It is possible that cannabis may have an effect in treating some types of cancer. But it may make other types of cancer grow or spread more quickly.

Research has looked at how using cannabis may affect the risk of developing cancer. This has produced different results. Some studies show that using cannabis may help reduce the risk of developing cancer. But there are also studies that show it increases the risk of cancer.

Because of these mixed results, experts feel that there needs to be more research into the chemicals found in cannabis and their possible benefits.

Cannabis oil and CBD oil

Many types of cannabis oil are sold online. CBD oil is also sold in some shops, such as health food shops.

Cannabis oil can contain different amounts of THC and CBD.

CBD oil in its pure form does not contain THC. This means it does not have any psychoactive effects (it does not make you feel stoned). CBD oil is not a controlled substance under the Misuse of Drugs Act. It can be sold in the UK as a food supplement and if the seller does not make any claims about its medicinal properties.

Understandably, many people want to try anything that may help treat cancer, particularly if things are not going well with conventional cancer treatments. This could be if they have cancer themselves, or if a loved one has cancer.

But if you are thinking about using CBD oil, there are some important things to consider:

Effects on cancer treatment

It is important to talk to your cancer doctor before using CBD oil. CBD can affect some cancer treatments, and your cancer doctor needs to know about any possible effects or side effects you might have.

Buying cannabis products online

Buying cannabis products online can be risky. As they are not regulated, it is often difficult to known the quality and what they contain. In 2017, a researcher tested 84 CBD products sold online. The results showed that:

  • about 1 in 4 (26%) contained less CBD than stated on the label
  • almost half (48%) contained more CBD than stated on the label
  • more than 1 in 5 (21%) contained THC.

Side effects

There are side effects of using cannabis oil. THC can:

  • increase your heart rate
  • cause dizziness, hallucinations and paranoia
  • make you feel stoned.


THC and products that contain THC are illegal in the UK.

Effects on other medicines

CBD and THC can affect how some medicines work. Talk to your healthcare team if you have questions about this.

Finding reliable information online

If you are looking for information online, you want to be sure that it is accurate and up to date.

Here are some things to think about when you are looking at a website:

  • Is the information regularly updated? You should be able to find the date when the information was last reviewed on each page.
  • Is it clear who has written the information? A good website should tell you about the organisation that has made the pages or written the information.
  • Are there references? The website should list its sources of information, or tell you who to contact to find out.
  • Is the website sponsored by a company? If so, it might mean the information is biased towards that company's products or services.

You can find more detailed information about cannabis on the National Cancer Institute's (NCI) website and Cancer Research UK's website.

About our information

  • References

    Below is a sample of the sources used in our complementary therapies information. If you would like more information about the sources we use, please contact us at

    Cassilieth B. The Complete Guide to Complementary Therapies in Cancer Care: Essential Information for Patients Survivors and Health Professionals. 2011. 

    Ernst E, et al. Oxford Handbook of Complementary Medicine. 2008. 

  • Reviewers

    This information has been written, revised and edited by Macmillan Cancer Support’s Cancer Information Development team. It has been approved by Dr Saul Berkovitz.

    Our cancer information has been awarded the PIF TICK. Created by the Patient Information Forum, this quality mark shows we meet PIF’s 10 criteria for trustworthy health information.