In my research, I could not find any credible, peer-reviewed medical literature that concludes that marijuana use in sporting activities provides significant performance enhancement or is potentially dangerous to the user or competitor. There are articles that discuss the affects of prolonged smoking of marijuana on the lungs, but these issues do not appear to have any relationship to sports.
I do not condone the use of marijuana in athletes - or anyone else, for that matter. I do not smoke tobacco, marijuana, take recreational drugs or drink alcohol. But these are all personal decisions and should not influence anyone other than members of my immediate family.
With that said, I find it inconsistent that sports regulatory bodies have chosen to test for and place marijuana on the banned substances list and not alcohol. I believe that the ban on marijuana use in combat sports is an attempt at legislating morality and not a safety or competitive integrity issue.
Dr Johnny Benjamin writing at MMA Junkie.