I just remembered. When I was going through my problems at work my boss consulted his attorney about the chaos my drinking was causing. We were very close, but he needed guidance on what his rights or obligations were. He was told he had to give me the opportunity to seek help, since it was a medical condition. (ADA-Americans with Disabilities Act may have said that too.) Of course, any time off would not necessarily be paid for. Once I sought help, though, he was told if I continued to drink, miss work and have a poor performance - causing others to pick up the slack - I could be let go.
Also, about whether or not to tell you've had a problem in the past - it's a Catch 22. No one wants to say anything negative about themselves when they're looking for work (I never have admitted it) - but if they find out later that you lied on your application, you could be terminated for that.