“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 than did MS. Males and females TS had a greater probability of developing lung cancer compared with NS. Males and female MTS had a slightly higher probability of developing lung cancer than did MS.

This difference was statistically significant: chi2 = 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 than did TS. This difference was statistically significant: chi2 = 71.61, p = .00003, with a correlation coefficient of 0.61, Z = 5.06, p < .05.”

https://www.ncbi.nlm.nih.gov/pubmed/19004418

http://www.tandfonline.com/doi/abs/10.1080/02791072.2008.10400641