The poor bioavailability of drugs, when given orally is due to:

Ans: a (1st pass metabolism) Ref: KDT, 6th ed, p. 291st pass metabolism refers to metabolism of a drug during its passage from site of absorption into systemic circulation. All orally administered drugs are exposed to drug metabolizing enzymes in the intestinal wall and liver (through the portal vein).The poor bioavailability of some drugs are due to high 1st pass metabolism.Transdermal or inhalational drugs have limited magnitude of lst pass metabolism (pie systemic metabolism).Feature of drugs with high 1st pass metabolism1) Oral dose higher than sublingual or parenteral route2) Bioavailability increased in patients with liver disease4) Bioavailability of a drug is increased if another drug competing with it in 1st pass metabolism is given concurrently e.g., chlorpromazine and propranolol