You might take him aside and suggest that he try a program of abstinence and recovery (NA, for example) if he's serious about keeping his job. Explain that if it continues, you'll have to let him go (and be ready to follow through with it)
Drug use can not only cause you a liability, but can endanger the others around him because of his mental state. What may seem to him like a good idea could injure or kill someone else.
You may end up with an employee that's better than ever...and all the time!
You're not doing yourself any good by keeping an employee that's on dope, and maybe just as or maybe even more important, you're not doing him any good by enabling this behavior. He'll never seek help if he's comfortable with the way things are.
Good luck
