What recreational drug can cause someone to be distant from friends and family and sexually with a mate?

My friend is using some type of recreational drug. When I asked him he says its just weed. Hes so withdrawn from everyone. Then he seems sleepy and fatigued a lot. He forgets a lot! One time I had to rush over his place and take him to work because he was too tired to get up for work. His moods are up and down. One time he came over my place and forgot his shoelaces. Sometimes he would ask me to drive him somewhere and he would purchase some drug from a guy standing on the corner. He says its "weed." Another time I drove him 20 miles across town because he said he wanted his "weed." One time he even took my car and did not ask me. In that case he said he just went to the store. My concern is that hes using something more serious than weed. He does not seem to have an interest in anything but watching sports and doing his drug. Usually I see him smoking weed(or this is what he lets me see him doing). Nor does he have a sex drive or the interest to keep in touch with me as a friend or even his other friends. I told him that he needs rehab. He said maybe you're right. I'm stuck! Should I even try to get him out of this stage or should I just back away and let him figure it out.