Cannibis can cause psychosis in some people, just type "cannabis psychosis" into google scholar.
The common rebuttal against this is "those people were vulnerable to psychosis from the get-go", but what does that mean when, without cannabis use, those persons might have gone their whole lives without a psychotic episode?
Edit: if you carefully read my post you might notice I'm not saying cannabis should be illegal.
So can alcohol, plus a lot more other very dangerous side-effects. Also, while not psycho-active, tobacco, coffee and even chocolate are very addictive substances and high-risk for some groups of people, too. So while technically true that cannabis is not without it's risks, it's not a valid argument against it unless they plan on banning half of the food that we eat...
So? Any action you take (including doing nothing) can have negative consequences. As long as it doesn't spontaneous cause psychosis in bystanders, let em smoke some pot.
The common rebuttal against this is "those people were vulnerable to psychosis from the get-go", but what does that mean when, without cannabis use, those persons might have gone their whole lives without a psychotic episode?
Edit: if you carefully read my post you might notice I'm not saying cannabis should be illegal.