I just say I don't drink. Lots of people don't drink. If they push it then that's just all the more reason to decline the invitations. At this point I'm comfortable "having drinks" with people, but I'm no longer tempted to drink alcohol. I order club soda and lime, and if they ask I just say I don't drink. If they ask why I just say, "I'm not good at it." That tends to throw people and they shut up. I say the same thing about weed. And it's true enough.