I have a friend whom I have known for 5 years. He has always been a happy and outgoing guy but he has been acting different recently. He never hangs out with his friends. He is always in his dorm even on weekends. He looks miserable in class and does not take care of himself anymore. When I asked him if anything was wrong, he just said "hmmm no" and left. Is this a sign of addiction? I know that he smokes marijuana but I am afraid that he is doing other serious things. Thanks.