Does smoking cannabis cause cancer? | Cancer Research UK
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Does smoking cannabis cause cancer?


The link between cannabis and cancer

Whether cannabis can cause cancer is not a simple question to answer. We know from very clear evidence that tobacco causes cancer. But we are still learning about the effects of cannabis and whether it has the same effects as tobacco. There are a number of factors that suggest that cannabis could cause cancer

The substances in cannabis smoke

Cannabis smoke contains many of the same cancer causing substances (carcinogens) as tobacco - at least 50 of them. In addition, cannabis is often mixed with tobacco when smoked.

One of these carcinogens is benzyprene. Benzyprene is in the tar of both tobacco and cannabis cigarettes. We know that benzyprene causes cancer. It alters a gene called p53, which is a tumour suppressor gene. We know that 3 out of 4 lung cancers (75%) occur in people who have faulty p53 genes. The p53 gene is also linked to many other cancers.

Cannabis also contains a substance called THC (tetrahydrocannabinol). It is the THC in the cannabis that changes your mood and behaviour. The amount of THC in a cannabis cigarette varies considerably. Researchers have shown that THC causes benzpyrene to promote the p53 gene to change.  But other researchers have looked at the effects of pure THC on brain tumour cells and found that it killed them in laboratory tests.  This is a long way from using it as a treatment.  But you can see from this that the evidence on cannabis causing cancer is confusing.

The way people smoke cannabis

People inhale cannabis smoke for longer than cigarette smoke. This is to get the full effect of the cannabis. But it means that the smoke is in contact with the lungs for longer.

Smokers’ lifestyles

Many people who smoke cannabis smoke tobacco cigarettes and drink alcohol too. Research evidence suggests that the combination of smoking tobacco, using cannabis and drinking alcohol can increase your risk of developing a cancer even further.


Evidence on cannabis and cancer

Several research studies have shown a link between cannabis and cancer. But other studies have shown no link. This makes it difficult to say exactly what the risk is. There have been a couple of systematic reviews that have tried to draw some conclusions on this.

In 2005 a review looked at the results of several studies into marijuana use and cancer risk. The researchers looked at 2 cohort studies and 14 case control studies. The case control studies involved many different types of cancer. Results were mixed and the researchers could not make any firm conclusions about the risk of cancer. It was also difficult to draw conclusions because of limitations in the studies. They included small numbers of people, involved too few heavy marijuana users and possibly underreported marijuana use in those countries where it is illegal.

In 2006 a systematic review looked at marijuana use and lung cancer risk. Although they could not find a significant link between marijuana and cancer, the reviewers reported that smoking marijuana increased tar exposure and caused changes to the lining of the small tubes in the lungs. They recommended that, until we have more definite evidence, doctors should warn people of the possible harmful effects of marijuana smoking. A New Zealand study in 2008 compared people with lung cancer to people who did not have lung cancer and found that regular cannabis use does increase the risk of lung cancer.

In early 2006 doctors reported on a possible link between cannabis and bladder cancer. Smoking is one of the main causes of bladder cancer. This study looked at men with bladder cancer under the age of 60, who had smoked marijuana, and compared them to men who hadn’t smoked it. The study showed that marijuana may be a possible cause of bladder cancer. But as the study was small, researchers need to investigate further to find out for certain.

A 2009 study showed an increase in risk of testicular cancers in cannabis smokers compared to non cannabis smokers.  The researchers say there was still an increase in risk after they accounted for tobacco and alcohol use.  But the study was too small to draw any definite conclusions, so we still need more research into this.

Two American studies found that cannabis seemed unlikely to increase cancer risk.  One, in 2006, found that there was an increased risk of cancers of the upper airways and digestive system (for example, the mouth, throat and food pipe).  But when they adjusted the data to account for smoking cigarettes and other common risk factors, they found that the link with cannabis disappeared.  In their data, it didn't seem to be the cannabis that was increasing the risk, but other factors such as smoking tobacco.  They concluded that if cannabis did affect cancer risk, the effect was likely to be small.  The other study, in 2009,  looked at head and neck cancers.  They found that risk of head and neck cancers in smokers and drinkers seemed to be lower in people who smoked cannabis as well.  But this is only one study and we would need more research to show whether this was a reliable finding or not.

Finally, there is laboratory research looking at the effect of some chemicals in cannabis smoke on cancer cells.  There is evidence that some of these substances can kill prostate cancer, breast cancer and brain tumour cells in the lab.  The researchers do point out that using these pure substances in the lab is very different from smoking cannabis.  They used far higher concentrations of each substance in their tests than you could get from smoking cannabis.

So at the moment we don't have clear evidence either way. We do know that smoking is unhealthy.  And that, like tobacco, cannabis contains cancer causing substances. Therefore it would seem likely to increase cancer risk. But we need more research to know this for sure.


Why researching cannabis is difficult

There are difficulties in researching the effects of cannabis. Many people who smoke cannabis also smoke tobacco. And users of cannabis often mix it with tobacco. This can make it difficult to know whether it is the tobacco, the cannabis, or both that has caused a cancer.

The amount of THC in cannabis also varies. Some of the cannabis available today is much stronger than it was 20 years ago. These versions contain more THC.

Another difficulty researchers have is in recruiting people who smoke cannabis into studies. Because cannabis is an illegal drug in many countries, people may be reluctant to take part in research. And if they do agree to take part, they may not say how much cannabis they actually smoke.

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Updated: 18 April 2013