Many times, because they don’t know what else to do, people use drugs as a way of medicating themselves out of depression or anxiety. In the beginning it seems to work, but eventually the effectiveness diminishes.
To try to recapture the earlier benefit, people will use more and more of the drug, which backfires. Their lives fall apart, and the depression and anxiety come back in full force.
Pot in and of itself can cause loss of motivation, increased obsessiveness, panic, anxiety, loss of memory and of course weight gain.
Pot does not cause schizpophrenia, but if someone is vulnerable to schizophrenia, it sure can bring it on, make it worse, and prevent recovery. Use of drugs can be the person’s way of trying to medicate the early symptoms.
If you are close to him or her, start going to Alanon, which helps those who come from dysfunctional families or who are close to a substance abuser. Also see if you can get your friend into counseling and a 12 step program. If he ‘gets it’ he will thank you down the line.