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Paper I — 2025 March (Supplementary (SAY)) (2019 Scheme) — Q13

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Paper I
2025 March (Supplementary (SAY)) (2019 Scheme) · 100 marks · 180 min

Question

Briefly describe kinetics of elimination.

Q134 marksShort Answers

Answer

Drug elimination follows one of two kinetic patterns:

First-order kinetics — a constant fraction of the drug present is eliminated per unit time (rate of elimination is proportional to plasma concentration). Most drugs at therapeutic doses follow this pattern; plasma half-life is constant and dose-independent, and a plot of log plasma concentration versus time is linear.

Zero-order kinetics — a constant amount of drug is eliminated per unit time, independent of plasma concentration, because the eliminating enzyme/transport system is saturated. Occurs when a drug’s therapeutic dose approaches or exceeds the capacity of its metabolizing enzyme — e.g. ethanol, phenytoin (at higher/toxic doses), aspirin (in overdose). Half-life is not constant (it increases with dose), and small dose increments can produce disproportionate rises in plasma level — the mechanistic basis for the narrow therapeutic margin and unpredictable toxicity of drugs exhibiting zero-order kinetics at clinically used doses.

Some drugs shift from first-order to zero-order kinetics as dose increases (dose-dependent/nonlinear kinetics), once the eliminating pathway becomes saturated — phenytoin is the classic clinically important example.

  1. 1Pharmacokinetics

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