I agree with many on this thread when I say that I think it is a bad idea to meddle with any drugs when playing. Personally, though, I prefer not to take even paracetamol when I have a headache never mind to stop any other pain. Just a personal preference...

Most painkillers will not stop pain applied externally so there would not be any effect if you took them for play purposes. And you really should not take such things that casually. Paracetamol in particular can cause problems for the liver if taken to excess and I know of several people who are in fact physically addicted to drugs like Ibuprofen to the extent that they cannot stop taking it without suffering extreme migraines. The body can adapt to drugs so that, in order to get the same effect, you have to take a higher dose and higher doses can lead to more side effects and more damage.