Smoking marijuana prevents cancer: the controversy starts here

22 January, 2009

An interesting article published in the second half of last year went largely uncommented (to my knowledge), despite its potential to “ignite a media firestorm”. It went unnoticed because it was published in an academic journal (popular journalists usually begin and end their research with the latest issue of Popular Science), and because drug researchers in the United States have had enough of being the whipping boy for each “unhelpful” piece of research that is allowed to emerge from their tenuously-funded labs.

I am drawing some small attention to it here because I think such an unusual finding should not be hidden from view. However, I should caution the unwary with one of the most fundamental principles for assessing scientific literature: ONE STUDY PROVES NOTHING. There have been plenty of medical stories published on slow news days that rely on one single paper (or sometimes just a press release), which may be statistically or methodologically flawed, biased, ignorant of relevant discoveries in related research, or just lumbered with a misleading abstract (summary). Only repeated examination by multiple researchers and reviewers will allow us to draw a strong conclusion one way or another.

A.L.C. Chen, T.J.H. Chen, et al. (2008). Hypothesizing that marijuana smokers are at a significantly lower risk of carcinogenicity relative to tobacco-non-marijuana smokers: evidenced based on statistical reevaluation of current literature. Journal of Psychoactive Drugs 40(3): 263-272.

ABSTRACT

A hypothetical link between marijuana smoking and cancer has been established based on a number of misleading assumptions. However, recent studies tend to suggest, if anything, an inverse association between marijuana use and cancers. To test the hypothesis that marijuana smoking significantly lowers the risk of developing cancer in humans, we analyzed published data from a prospective cohort study on cancer incidence among nonsmokers (NS), marijuana-only smokers (MS), tobacco-only smokers (TS), and marijuana and tobacco smokers (MTS). Using the log linear model to calculate the probability of developing each cancer form as a function of the interaction between marijuana and tobacco smoking, as well as functions of marijuana and tobacco smoking main effects whereby chi square statistics were calculated for the interaction and main effect estimates, we found that in all cases tested there was a significantly lower risk for MS compared to TS. Male and female TS had a greater probability of developing lung cancer (r = 0.72) than did MS (r = 0.02). Males and females TS had a greater probability of developing lung cancer (r = 0.72) compared with NS (r = 0.05). Males and female MTS had a slightly higher probability of developing lung cancer (r = 0.73) than did MS (r = 0.07). This difference was statistically significant: c2 = 30.51, p < .00001, with a correlation coefficient of -0.75, Z = -7.84, p < .05. Male and female MTS had a lower probability of developing lung cancer (0.23) than did TS (0.77). This difference was statistically significant: c2 = 71.61, p = .00003, with a correlation coefficient of0 .61, Z = 5.06, p < .05.

The study they examine is in:
S. Sidney, C.P. Quesenberry Jr, G.D. Friedman, I.S. Tekawa (1997). Marijuana use and cancer incidence. Cancer Causes and Control 8(5): 722-8.

As the authors say in their conclusion, “our re-evaluation of a large cohort study indicates that marijuana smokers are at a significantly lower risk of cancer relative to those who smoke tobacco and not marijuana.”

What the authors do not dare to state in their abstract or conclusion is that Marijuana-only smokers have a lower probability of developing lung cancer than Non-smokers. This information is available within the body of the paper (p.269): “For all sites [i.e. all cancer sites examined: colorectal, lung, melanoma, prostate, breast, cervix], males and females had a lower probability of developing cancer if they smoked marijuana and not tobacco (r = 0.13) than if they smoked neither marijuana nor tobacco (r = 0.32).” However, in the discussion, they do suggest that “smoking marijuana may treat lung cancer at its roots early on, while treatment still matters”.

They also cite a number of animal studies to support their thesis, and they assert that, unlike for tobacco smoking, there is no convincing epidemiological link between marijuana smoking and cancer. The role of adulterants in commercially-sold tobacco, as opposed to illegally-produced marijuana, is not considered here.

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