I added this too..

Causes people to fail school, or be dumb.
This is a tough one as there is not a particularly great way of testing it. But science has suggested that if used responsibly it will have no effect on life. I can also find countless examples of extremely smart marijuana users, that do not let it effect there lives at all. Yes, some people do use cannabis in wrong ways, but it can also be a good thing if used responsibly.