I think the secret clues are in what he's not saying. First, he never said it was a prescribed medicine. My bet, since we already know he was day drinking, is that it was a recreational substance.

Second, he never actually says he got vomit on his pants. He just says that the mix "eventually" made him vomit. It's likely that he was so messed up from his booze and pharmaceutical cocktail that he shit himself and he's leaving that part out.

That or he just ditched his pants to rub one out, which is where I'd put my money.

Right, this clearly is not Mom's spaghetti