Taking daily recommended dosages of ibuprofen and acetaminophen caused a substantially greater increase over placebo in the amount of quadriceps muscle mass and muscle strength gained during three months of regular weight lifting, in a study by physiologists at the Human Performance Laboratory, Ball State University.
They don't know why this happens but the author seems to indicate that it's some sort of metabolic process that allows for more muscle mass. Could be. But I'd argue that reducing inflammation and thus damage between workouts is a big factor too. For years I've avoided the temptation to take ibuprofen after hard workouts because I didn't want my body to be relieved of the stress. My thinking has been that the only reason for working out is to create stress on the body so that the body responds by rebuilding itself stronger. By taking something to reduce the stress, by my thinking, you're reducing the effective training load. Maybe I'm wrong. Maybe I'm not.
A few key points: 1) Don't mess around with ibuprofen. It's not worth the liver damage people. 2) Really don't mess around with acetaminophen... it's actually one of the riskier drugs in your bathroom because the threshold between safe and very not safe is narrow and positioned close to safe. 3) The studies weren't in any way focused on endurance athletics. They were done on people aged 60-78 who are already losing a lot of muscle. We've frequently seen things that help weight lifters but hurt endurance types.
So I certainly won't be changing my ibuprofen policy. But, since ibuprofen is one of the only things that helps get rid of my migraines when I get them, maybe I'll feel a little less bad about having to take it from time to time.