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2021 December (Supplementary) (2010 Scheme) · 40 marks · 120 min

Question

Plasma half life

Q33 marksShort Notes

Answer

Plasma half-life (t½) is the time taken for the plasma concentration of a drug to fall to half its original value.

First-order kinetics

  • t½ = 0.693/Ke; constant regardless of dose
  • After ~4–5 half-lives, drug is essentially eliminated and steady state on repeated dosing is achieved

Clinical significance

  • Determines dosing frequency/interval
  • Determines time to reach steady state
  • Determines time for a drug to be essentially eliminated after stopping it
  • In zero-order kinetics (e.g., phenytoin, ethanol), half-life increases with dose

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