Most prescription medicines are for physical problems. The reason they're only available via prescription is that they could have unanticipated side effects and their use should be monitored by a physician.
Prescription meds used to control mild-to-medium pain probably don't interfere with acting skills.
You're wise to be wary of "mood-altering" drugs, whether prescribed, over-the-counter, or obtained "on the street". Even the caffeine in coffee can affect your emotional state.
Bottom line: Just because a medicine is only available by prescription, that doesn't mean you should automatically avoid it. If you're prescribed something, there's a reason. However, before you get the prescription filled, look it up on the Internet, where you'll be able to read about any side effects or dangers. Then you can make an informed decision.