Does a person on drugs talk to themselves constantly?

No not necessarily. Many a times people are experiencing very high levels of stress which causes such disorders. Generally starts when someone keeps thinking about someone giving them a hard time or some difficult situation they are facing and they are at the recieving end of an emotionally draining problem. He/she does not get an outlet and often when they are alone, all anger that stays in thier thoughts comes out in words and actions. My advice, this person needs to relax. Probably he is feeling helpless and doesn't know what to do. Try asking him, try listening to what is he talking about. Try asking him seek some counselling. Try discussing this situation with other confidentes. Go seek help somewhere. It will get worst. Next stage could lead to violent outbursts, memory losses, severe depression...leading to dependency on perhaps drug. Seek help ASAP, its serious. Good Luck.