Or maybe it's the alcohol, menthol, D&C Red 33, FD&C Red 40, monoammonium glycyrrhizinate (which is some sort of licorice extract?), polyethylene glycol, sodium benzoate, sodium carboxymethyl-cellulose, sodium citrate, sodium cyclamate (gross!), sorbitol, or sucralose. Or the horrifying fake cherry flavor (although the Straight Dope claims there's a reason for that).
Oh, and BTW, the alcohol? I was not aware it was in there. I am allergic to alcohol (or at least as allergic as people get). And when I say alcohol allergy, I go straight from "Oh look, 0.05% fake wine" to alcohol poisoning.