Where the protagonist is supposed to be a rough person, I think it serves to reinforce the idea that they're hurting from the ass whipping they just dealt/received, and don't have time for a hospital or inclination to read the dosage instructions.

Normal people in the States that I know don't typically numb pain with a handful of opioids. Of course, that's not to say they (and their over-prescription) haven't been an issue for people with chronic pain.