Paper I
2019 July (Supplementary) (2010 Scheme) · 40 marks · 120 min

Question

First pass metabolism

Q33 marksShort Notes

Answer

First-pass metabolism (presystemic metabolism) is the extensive metabolism of an orally administered drug by the liver (and sometimes gut wall) before it reaches the systemic circulation, since orally absorbed drugs are carried via the portal vein directly to the liver.

Consequences

  • Significantly reduces oral bioavailability of drugs with high first-pass metabolism (e.g., propranolol, lignocaine, glyceryl trinitrate, morphine)
  • Requires higher oral doses than parenteral doses for equivalent effect
  • Can be bypassed by sublingual, IV, IM, rectal (partial), or transdermal routes
  • Shows marked inter-individual variation

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