One of either tylenol, or aspirin block pain receptors in the brain, if I recall correctly. Or do something complex with the brain chemical that transmits pain. Read a bit on it once, and I still think it's hard to believe.
Of course, if it -were- a placebo, they'd have to say -something- to make the masses think it worked. And keep money flowing into their pocket! Sometimes, perception may, or may not be more important than actual healing ability. As Protagonist said, the mind is a powerful thing.
(Article about a study thing on placebos: http://www.csicop.org/si/9701/placebo.html )
(Please note: Tylenol and Aspirin being a placebo is personal opinion only. 8p)
In either case, I vote for little green pills, and little blue pills in medkits! Heck, throw little purple pills in there too!
Doctor person: "Uh, ya, this little green pill's the Painmaster 2080! It'll fix that hangover right up. Rare in the 'dome. Just 500 chy, and it's all yours. Zeke over there took it, took his headache clear away."
Zeke looks over, and gives a thumbs up.
"Oh, Results may vary, of course."