I've got a friend who is 26 and he smoked weed pretty regularly since he was 16. About a year ago he had a mental breakdown and was diagnosed with schizophrenia. His family has a history of it, but a strange coincidence is of the six family members of his who have the condition, four of them were regular weed smokers. The other two weren't regular users but did smoke on occasion. Four of the six have managed to get it under control through the use of medication. All six use medication, but the two who still suffer the most are also still smoking weed. All are male.
Purely anecdotal, but very interesting to me. We still don't know the side effects of the drug and that's enough to keep me away.