Family.

If a member of a family starts taking drugs and gets addicted it would cause many problems. The parents could start to have more arguments which could end in a divorce. People would probably want to stay away from the family so they wouldn't get influenced. If the user is the child, they would lose their parents' trust especially if they had to steal money to buy the drugs. The community would possibly blame the parents for not raising a good child. The parents might send the child to a boarding school, rehab center, or keep them at home so they can make sure the child doesn't do anymore drugs.
Either way, if someone takes drugs he/she is not the only one who gets effected because everyone around them will effected in some way.