In November, four states – California, Massachusetts, Nevada, and Maine – passed measures legalizing marijuana for recreational use, bringing the total number of states with legal cannabis to eight, plus the District of Columbia. The vote in Maine was quite close and is expected to be challenged by opponents, but as of now, 26 states and the District of Columbia have legalized marijuana in some form, whether for medical or recreational use.
While marijuana use remains illegal under federal law, the expansion of state laws decriminalizing or sanctioning its use is likely to renew the debate over whether smoking marijuana increases the risk of developing cancer, particularly lung cancer.
As of now, scientific evidence on the question is mixed. On the one hand, smoke from marijuana combustion contains many of the same toxins and cancer-causing substances as tobacco smoke, including benzyprene, a potent carcinogen. And because marijuana users tend to inhale more deeply and hold their breath longer than tobacco smokers, marijuana may have more opportunity to damage lung cells. On the other hand, several studies haven’t found a connection between marijuana smoking and cancer of the lung or other organs. Part of the challenge in studying this subject is that many people who smoke marijuana also smoke tobacco, making it difficult to isolate what role, if any, marijuana plays in lung cancer development.
A review by the National Cancer Institute shows the inconclusive nature of research in this area. One analysis, which melded data from three studies of men in northwestern Africa, found a significantly increased risk of lung cancer among tobacco smokers who also inhaled cannabis (the plant from which marijuana is derived). By contrast, a large U.S. study of men aged 15 to 49 years found that cannabis use was not associated with tobacco-related cancers and several other common malignancies. The study did find that, among nonsmokers of tobacco, those who had used cannabis had an increased risk of prostate cancer. Along the same lines, a review of 19 separate studies of patients with precancerous or cancerous lung growths who inhaled cannabis concluded that the studies failed to show a significant link between smoking cannabis and lung cancer, after adjusting for tobacco use.
A similarly unclear picture emerges from research examining possible links between marijuana smoking and squamous cell cancers of the head and neck. A pooled analysis of nine studies involving more than 10,000 people found cannabis smokers had an elevated risk of oropharyngeal cancers and a reduced risk of tongue cancer, compared with those who never smoked cannabis.
Three large-scale studies reported an association between cannabis use and elevated risk of testicular germ cell cancers. However, the sample sizes in these studies were too small to adequately address issues such as the amount of marijuana smoked or how frequently or recently it was used.
Some studies have suggested that cannabis may protect against the development of certain types of tumors. One, conducted in mice and rats, found that as doses of the drug were increased, the incidence of certain types of liver cancer decreased. Another study found that one of the active compounds in marijuana, cannabidiol, exerts a cancer-preventive effect in mice. Some studies have suggested that cannabidiol may also increase malignant cells’ absorption of certain chemotherapy drugs.
The inconsistent and sometimes contradictory findings regarding marijuana smoking and cancer risk underscore the need for more research in this area, investigators say.