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Paper I — 2022 October (Supplementary) (2010 Scheme) — Q6

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Paper I
2022 October (Supplementary) (2010 Scheme) · 40 marks · 120 min

Question

Plasma half life

Q63 marksShort Notes

Answer

Plasma half-life (t½) is the time required for the plasma concentration of a drug to fall by 50% from any given level, for a drug following first-order kinetics.

Relationship: t½ = 0.693 × Vd ÷ Clearance — determined by both volume of distribution and clearance, not by either alone.

Clinical significance:

  • Determines dosing interval — a drug is generally dosed roughly every one half-life to maintain therapeutic levels within an acceptable range.
  • Approximately 4–5 half-lives are needed to reach steady-state plasma concentration on repeated dosing, and roughly the same number of half-lives for a drug to be essentially eliminated after stopping.
  • Drugs with a short half-life may need frequent dosing or a sustained-release formulation; drugs with a long half-life (e.g. digoxin, amiodarone) often need a loading dose to reach therapeutic levels rapidly, since waiting for steady state through maintenance dosing alone would take too long.

Zero-order kinetics exception: drugs like ethanol, phenytoin (at high doses), and aspirin (in overdose) are eliminated by a constant amount per unit time rather than a constant fraction, so they do not have a fixed, dose-independent half-life.

  1. 1Pharmacokinetics

Other Years Asked

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  18. 18First pass metabolism2019 July (Supplementary) (2010 Scheme)Short Note · 3 marks
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