Among men, I'd have to say masturbation. It's widely available, costs nothing, and gives the person a feeling of euphoria without as large of a long term effect as drugs. While on the front side, masturbating seems to have no negative effects, it has been proven time and time again to ruin relationships between the masturbater and those they masturbate to.
The plus side of masturbation is that it's much easier to stop then doing other drugs. Stopping masturbating is much more a battle of will against human nature then it is a battle against an addictive ingredient. All you have to do is want to stop masturbating, stay away from things that might usually turn you on and make you want to masturbate. Stay away from your phone when you go to the bathroom, or live with someone else you are not comfortable masturbating around.