May 13, 2009
A 1999 study showed a modestly increased risk of certain types of head and neck cancer among marijuana smokers. Due to methodological limitations, the researchers warned that their "results need to be interpreted with some caution in drawing causal inferences." But warnings about this alleged risk have shown up from time to time in materials put out by prohibitionist types, including the National Institute on Drug Abuse.
A new study, just published in the journal Cancer Epidemiology, Biomarkers, & Prevention, suggests this may have been a false alarm.
Researchers pooled data from five studies, totaling over 9,000 participants (nearly 30 times the number in the 1999 study) and found that the risk of head and neck cancer "was not elevated" among those who had ever smoked marijuana compared to those who hadn't. Notably, "there was no increasing risk associated with increasing frequency, duration, or cumulative consumption of marijuana ..."
The researchers note that, due to the small number of long-term, very heavy marijuana users in the studies, they can't rule out increased risk from such very heavy use. But it is striking that the overall cancer risk among marijuana smokers was slightly lower than nonsmokers, though not enough to be statistically significant. That was also the case in a major lung cancer study a few years ago. In the new study, there were some subcategories in which the lowered risk among marijuana smokers came close to statistical significance.
But don't expect mere data to put an end to hysterical claims that marijuana is more carcinogenic than tobacco.