I would advise you to simply keep the facts to yourself. Simply say that you're on a medication that doesn't mix with alcohol. If asked what the medication is for, then tell them it's for sleep issues.

The end. Meaning: don't tell them any more than that. Some people can say terrible things, especially if they're getting drunk. People sometimes come up with all sorts of stories to motivate others to drink with them. Don't fall into that trap! Simply stick to your guns & have fun being sober.