Pretty sure I already countered the usual health risk points but I'll do it again.
There's usually just a couple of health risks that people argue:
1. It is bad for your lungs, more carcinogens than tobacco ETC...
Counter-point: You don't have to smoke it. If it were legal it would be much easier to obtain in smokeless form. Pretty obvious if you light a substance on fire and inhale the smoke in your lungs it's gonna be bad for you.
2. It develops schizophrenia
Counter-point: I'm pretty sure that if you are susceptible to get schizophrenia from smoking it you wouldn't just get it instantly. It is something that happens fairly gradually and some of the people who get schizophrenia from it continue to smoke it after they have the disorder. And also it does not cause it, but only triggers it. Many other things could have triggered it later in those people's lives.
And then there's the study which Brawny linked to, which is just plain ridiculous. It even states in the article that the research proves nothing.