Acetaminophen (Tylenol) was found to increase power output and 10 mile time trial time (26:15 vs 26:45 on the placebo ride) among 12 cyclists. These findings support the notion that exercise is regulated by pain perception, and increased pain tolerance can improve exercise capacity. (J. of Applied Physiolgy)
According to medicinenet.com, acetaminophen is a pain reliever that can potentially harm liver in excessive doses or if you drink alcohol. The recommmended dose is 325-650 mg every 4-6 hours.
I wouldn't make it a habit, and I would not use it in a marathon (dehydration may increase concentration of the drug as your liver tries to clear it), however I might consider popping a couple of tylenol an hour before my next hard 5K.