That's a pretty loaded question, something you should probably refer to an Aviation Medical Examiner about, especially if it's in reference to something you are currently taking and aren't sure about. If you have to take medication for some kind of bad condition (for example) then it would probably be the condition itself and not the medication that would bar you from getting a medical. Again, best to find a few AMEs and get some opinions, or maybe even your local FSDO. As far as OTC medications, a general rule of thumb is don't take anything that has side effects such as drowsiness, but that should be common sense.