Actually, for some very unlucky individuals, cannabis can induce psychosis:
http://www.npr.org/sections/health-...tic-symptoms-for-some-but-do-the-effects-last
And, when I was young, around 16 or so, I smoked weed for a few months, and then also had some sort of meltdown after smoking with my friends as I had done so many times before... I obviously didn't hurt anyone or myself but, it was probably the worst experience I've ever had. I saw demons and was screaming for help and kept running up and down the stairs of the house I was in. I lost all sense of myself and literally lost all 5 senses: I couldn't see, hear anything but myself screaming, touch, everything was gone and I was in a nightmare. And I promised God that if it wore off I would never smoke weed again -- and I kept my promise. I cannot even begin to explain the horror of what I saw and felt, and none of my "friends" helped me. I begged them to call 911, to call my mom, to take me to the hospital, but they didn't do anything... It's fine, I guess, they were young too and probably scared but, I'm thankful I learned my lesson and never did any type of drug after that experience... Clearly, I'm one of the unlucky ones that marijuana severely negatively affects my brain.
I never developed any long lasting effects, I had some disassociation for awhile, but nothing bad and eventually got over it. What I'm saying is that weed can induce psychosis and it's awful.