Above all, never drink, and maintain your professionalism. There is no credit due for being a recovering anything in the workplace, so don't become a recovery missionary, or talk about your rehab stay with anyone that doesn't need to know about it. If any of your co-workers don't know, keep it that way, and don't volunteer unnecessary information.
If co-workers ask where you were, you can say that you had a personal medical matter to attend to, or something similar. If they ask about drinking, don't say that you are allergic, or that you are sober, which just broadcasts that you are an alcoholic. Say that you are driving, that you don't drink, or that your doctor advised that you quit drinking.
All your employer wants is responsible and reliable employees, so reassure your bosses, co-workers, and/or HR that you are back in the game, and that everything is under control. Do not violate any terms that you agreed to in exchange for leniency, such as aftercare, which will send the opposite message, and which could trigger more drastic sanctions.
Once again, maintain your professionalism.