Paper I
2016 October (Supplementary) (2010 Scheme) · 40 marks · 120 min

Question

First pass metabolism.

Q73 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 the oral bioavailability of drugs with high first-pass metabolism (e.g., propranolol, lignocaine, glyceryl trinitrate, morphine)
  • Requires higher oral doses compared to parenteral doses to achieve the same systemic effect
  • Can be bypassed by alternate routes of administration: sublingual, IV, IM, rectal (partial), transdermal
  • Drugs with high first-pass metabolism show marked inter-individual variation in bioavailability

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