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A recent report by the British Lung Foundation made the headline-grabbing claim that the risk of developing cancer was 20 times higher per marijuana joint than per cigarette. However, the scientific data simply do not support this contention.

Dr. Donald Tashkin, professor of medicine at University of California, Los Angeles, is among the foremost researchers studying the effects of marijuana on the lungs. His 2006 study, one of the largest to look at marijuana use and lung and upper-airway cancers, found that the “association of these cancers with marijuana, even long-term or heavy use, is not strong and may be below practically detectable limits.”

A larger, longer-term study published in 2012 by a separate group of researchers showed that marijuana had no detrimental effect on lung function. Tashkin, who was not involved in the study, called it “well conducted” and said the results confirmed his own findings.

A smaller recent study did find a link between extremely heavy marijuana use (at least a joint a day for 10 years) and lung cancer, but that study suffered several methodological flaws. For one, Tashkin explains, it included very few people who actually smoked marijuana heavily, making its results “imprecise.” (Unlike the average cigarette smoker, the average marijuana smoker doesn’t smoke daily — let alone daily for 10 years or longer.) Other researchers, pointing out the study’s various limitations, agreed that its results could have occurred by chance alone.

The tobacco literature, in contrast, is unequivocal: smoking increases the odds of developing lung cancer by a factor of at least 9.

It’s possible that marijuana may increase the risk of death from other causes, but so far, research hasn’t been able to offer any evidence of that. Again, in contrast, the research does show that smoking cigarettes is clearly linked with a doubling to tripling of heart disease and stroke risk.

If mental health is your concern, there is some evidence that marijuana can hasten or worsen schizophrenia in those who are predisposed to it. But over the past five decades, as marijuana use has increased, rates of schizophrenia haven’t risen in tandem. Further, there’s no evidence that current marijuana policy reduces the risk to those who may be vulnerable to developing the disorder: the 3,500% increase in the drug-fighting budget since the 1970s hasn’t stopped the majority of boomers and succeeding generations from smoking pot. Moreover, legalizing the drug could offer a way to reduce psychosis risk; by regulating the chemical balance in marijuana, its safety could be controlled. That can’t be accomplished if the drug is illegal.

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