UCLA's Dr. Donald Tashkin states he would be in favor of legalization of marijuana.  Why?  After all he did study chronic marijuana smokers and the effects of heavy long term marijuana smoking on the lungs with regards to lung cancer and COPD (chronic obstructive pulmonary disease).  Prior to the study he hypothesized there would be a link between heavy marijuana smoking and lung cancer or COPD.  Guess what?  He was wrong!  To his surprise the study concluded otherwise. 

"What we found instead was no association and even a suggestion of some protective effect," says Tashkin, whose research was the largest case-control study ever conducted. The study was funded by the National Institutes of Health.

Tobacco smokers in the study had as much as a 21-fold increase in lung cancer risk, and developed COPD more often in the study.  Researchers found marijuana did not impair lung function. (perhaps this is why Michael Phelps was able to win 8 gold medals in the Bejing Olympics... of course in all fairness he did win the medals prior to getting caught on camera smoking marijuana).

Tashkin, supported by other research, concluded that THC (tetrahydrocannabinol), an active ingredient in marijuana, has an "anti-tumoral effect" in which "cells die earlier before they age enough to develop mutations that might lead to lung cancer." 

"Early on, when our research appeared as if there would be a negative impact on lung health, I was opposed to legalization because I thought it would lead to increased use and that would lead to increased health effects," Tashkin says.

"But at this point, I'd be in favor of legalization. I wouldn't encourage anybody to smoke any substances, because of the potential for harm. But I don't think it should be stigmatized as an illegal substance.

"Tobacco smoking causes far more harm. And in terms of an intoxicant, alcohol causes far more harm," Tashkin stated.

And of course this is ONLY in reference to smoking marijuana.  It doesn't take into consideration the other methods of medicating, such as vaporizing marijuana and marijuana edibles.