It will probably not come as any surprise to anyone who has been in high school during the last forty years, or listened to Gary Johnson talk, that heavy pot use can make you stupid. It's effects on the brain are mediated mainly through the dopamine neurotransmitter pathways, where it imitates and blocks natural endocannabinoids, but how it's long term effects work remains poorly understood. At least that's what I took away from this research article in the latest issue of Nature.
The dopamine system is very widespread in the brain, so there are lots of places for effects to manifest themselves. Searches for long term changes seem to show more effect when exposure is early, especially during gestational development, but are complicated by apparent differences between mice and rats, and among different lines of mice, and of course, among people.
Is pot addictive? In some people, yes, among others, apparently not. Can it contribute to psychosis? Same answer. there doesn't seem to have been much success in teasing apart these effects or understanding what factors are involved.
Of course widespread legalization is likely to produce more research subjects.