it's basically taking extra doses of chemicals your body already produces. now when this is in the form of endorphines, or seratonin, you can get quite depressed. however I've not yet heard of anything particularily bad happening from excess cannabinoids. generally all the negative mental effects associated with use that I've experienced go away 100% with a few days of abstinence.

besides, I just started college and I've been keeping up with my work and remembering plenty of facts just fine.