79 different studies: they found only moderate evidence indicating that marijuana reduced nerve pain and pain from cancer.
If a pharmaceutical company, for example, wanted to get a drug approved for a medical condition and they only submitted anecdotal data, there's absolutely no chance that that drug would be approved.
Links
[1] http://www.npr.org/2015/06/24/417045126/review-raises-troubling-question-about-marijuana-s-safety-effectiveness
[2] http://rethinkpot.org/tags/medical-0