One point (thanks, squirrelboiler!) in response to Heath Ledger's death is that if you get all your meds from one doctor and one pharmacy, the computer will alert them to the possible drug interactions and they can counsel you about them. It is highly likely that actors and actresses get meds on location when they're filming and that they fall through this safety net. In that case they should keep a list of medications that have been prescribed for them, whether they are currently using them or not, so the new doctor can tell them not to take some meds together.
I also read recently that the popular herb St. John's Wort can't be taken with several commonly used prescription drugs because it concentrates their ingredients. Be sure to list supplements and herbs if you keep a list of medications for your doctor.