Scientists who conducted a 20-year study said this about the drug that the U.S. has deemed as a Schedule I Controlled Substance with no medical value, while cigarettes and alcohol are still legal:

Marijuana may have beneficial effects on pain control, appetite, mood, and management of other chronic symptoms. Our findings suggest that occasional use of marijuana for these or other purposes may not be associated with adverse consequences on pulmonary function. It is more difficult to estimate the potential effects of regular heavy use, because this pattern of use is relatively rare in our study sample; however, our findings do suggest an accelerated decline in pulmonary function with heavy use and a resulting need for caution and moderation when marijuana use is considered.

Remember, tons of people are in jail draining the state and federal coffers for being in possession of this substance.