Smoking Marijuana Doesn’t Appear To Increase Cancer Risk

Marijuana plantsOver the years, many scientific studies have looked at whether marijuana smokers are at an increased risk of developing cancer. Interestingly, despite the fact marijuana smoke shares many of the same carcinogens as cigarette smoke, no study has found a significantly increased risk of cancer as a result of smoking marijuana.

The active ingredient in marijuana, Tetrahydrocannabinol or THC, has not shown mutagenic or carcinogenic activity in any laboratory studies and in fact THC has shown some promise as a potential anti-cancer treatment. One study, published in 2007 found that THC cut the growth of lung cancer tumours implanted in mice by around 50%. On the negative side, studies suggest that smoking a marijuana joint deposits more tar in the lungs than a cigarette does. Furthermore, the smoke released when marijuana is burnt shares many of the same carcinogenic compounds as tobacco smoke including phenols, vinyl chlorides, aldehydes, nitrosamines, and polycyclic aromatic hydrocarbons. Therefore one might expect that an increased risk of cancer, if one exists, would occur predominately at sites in contact with inhaled marijuana smoke such as the mouth, throat, and lungs.

A 2006 study, published in the journal Cancer Epidemiology, Biomarkers & Prevention , looked at the risk of cancers of the lung and upper aero-digestive tract in marijuana smokers. Although the researchers found an increase in the crude rate of cancer for heavy marijuana smokers, after adjusting for cigarette smoking and other confounders the association disappeared. Among the heaviest marijuana smokers who had smoked more than 21,000 joints over their lifetime, the relative risk of lung cancer compared to never smokers was 0.62 while the relative risks for laryngeal and oral cancer were 0.84 and 1.1 respectively. Compare this to heavy cigarette smokers who have relative risks of about 12, 20, and 15, for lung, laryngeal, and oral cancer compared to non smokers. The researchers concluded that: “contrary to our expectations, we found no positive associations between marijuana use and lung or UAT cancers”.

A second study, involving more than 60,000 members of the Kaiser Permanente Medical Care Program in California looked at the overall risk of cancer amongst marijuana smokers. Male marijuana users had an overall cancer RR of 0.9 compared to non users while female users had a RR of 1.0. Once again, the researchers concluded that marijuana use was not associated with overall cancer risk.

One of the important differences between marijuana and cigarette smoking is that marijuana users typically smoke a much smaller number of joints over their lifetime than a tobacco smoker would smoke cigarettes. The average marijuana user smokers 1-2 joints a day compared to 20+ cigarettes a day for the average cigarette user. Due to its non-addictive nature, marijuana smokers are also more likely to quit smoking and will typically do so in their late 20s to early 30s whereas a large percentage of cigarette smokers will continue to smoke for their entire life. It is therefore possible that the lack of association between marijuana and cancer can be explained, at least in part, by lower exposure levels.

Nevertheless, marijuana users who don’t smoke cigarettes can probably rest easy – any increased cancer risks from marijuana smoking are almost certainly a fraction of the increased risk faced by cigarette smokers.