If you say, "medication," be ready for the, "Oh, what kind of medication?" question(s.)
I've learned to keep it simple. "I don't like how it makes me feel," is something I'll bet every drinker has felt at one time or another. "I don't like it," is pretty non-arguable, too.
You'll be fine.
New meet and greets are usually awkward, regardless. If they're combined with alcohol 1.) People have no filter, so be ready and 2.) They won't remember whatever reason you give anyway. 3.) They likely won't even care.
The worrying usually has more to do with not understanding how to set boundaries. It's really no one's business about your past drinking unless you want it to be. That won't stop them from probing (especially if they're drinking and looking for drama) but you can say, "No," to the probing too.