We learnt some about this subject at biology class some years ago and as far as I remember, regular dozes of Marijuana is believed to decrease your learning ability by far. Secondly this drug is highly addictive and the need for a rush can sometimes lead to the use of far worse drugs (heroine, speed and whatever).

There are some other factors like the possibility of a psykosis (not sure how to spell that) but I cannot remember all the details.

Cheers