How smoking cause 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.