According to a study published in Journal of the American Medical Association, marijuana does not impair lung function and can 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 maybe due to taking a deep breaths while inhaling the drug and not from a therapeutic chemical in the drug.