Question: What is meant by first-pass metabolism? Need Assignment Help? Question options: A certain percentage of an absorbed drug may be altered in the liver before it enters the bloodstream and reaches its site of action. A drug's bioavailability is determined by it's first pass through the serum or plasma. Drugs that are administered sublingually must first pass through the digestive system and to the liver prior to circulation. A drug must first be passed to and bound to proteins prior to metabolism.