Cannabis is a psychoactive drug, so like chronic said you can only become addicted to it psychologically. It's a good choice to mentally prepare before a good smoke, not if you have woke up with a hangover, have an empty stomach and your metabolism is on crutches and the first thing you decide to do every day is get high - not such a good idea. So yeah, make sure you body is up to it, do some exercise, have a good meal, do some things and then chill out with a spliff at the end of it, can't go wrong. You body will be a lot more up to filtering out any 'toxins' in the bloodstream.
As far as Mental illness is associated with smoking, that's just unfortunate if you are prone to that type of thing. The induced introspective effects from smoking weed, if excessive, is where you gonna find paranoia.
My advice to anyone who smokes weed is to learn about how it effects the human body, that way any negative side-effects can be grasped and understood, and even thwarted through knowledge and understanding. For if something is gonna come about that is bad you gonna be more prepared for it, call it foresight.
Read; This is Cannabis by: Nick Brownlee. It has lots of information.