There’s a fair amount of evidence that marijuana does no harm to the lungs, unless you also smoke tobacco. One study published in Journal of the American Medical Association found that not only does marijuana not impair lung function, it may even increase lung capacity.
Researchers looking for risk factors of heart disease tested the lung function of 5,115 young adults over the course of 20 years. Tobacco smokers lost lung function over time, but pot users actually showed an increase in lung capacity.
It’s possible that the increased lung capacity may be due to taking a deep breaths while inhaling the drug and not from a therapeutic chemical in the drug.
The smokers in that study only toked up a few times a month, but a more recent survey of people who smoked pot daily for up to 20 years found no evidence that smoking pot harmed their lungs, either.
The National Academies report said there are good studies showing marijuana users are not more likely to have cancers associated with smoking.