Most drugs are metabolized primarily by the liver, meaning they have the potential to damage it, yet millions of people take multiple rx. A normal acetaminophen or Benadryl (diphenhydramine) dose should not add significantly to the risk of Rilutek hepatotoxicity, if liver function is normal at baseline and checked regularly. As Katie noted, it's silly to take Tylenol PM for sleep if you don't need pain relief. However, liver injury can manifest at any time and so a normal followup test doesn't mean that liver injury will never occur while on Rilutek. Hepatic injury is cumulative and can be caused/exacerbated by many things, including alcohol, hepatitis and recreational drugs. It is often the combination of lifestyle and rx/OTC that causes injury.