Drug metabolism and the cytochrome P450 system
Phase I and phase II metabolism, the main CYP isoenzymes with their substrates, inhibitors and inducers, the time course of inhibition and induction, prodrugs and toxic metabolites, and the interactions Paper 1 builds questions around.
Tier 1Medical sciences concept
Written with AI assistance and fact-checked against the sources below. For exam study, not clinical decisions. How notes are made
At a glance
- The liver converts lipophilic drugs into water-soluble products the kidney can excrete. Phase I (mostly cytochrome P450) adds a reactive group; phase II conjugates it.
- Six CYPs handle most drugs: 3A4 (about half), 2D6, 2C9, 2C19, 1A2 and 2E1.
- Inhibition raises substrate levels within days. Induction lowers them over 1 to 2 weeks, and the effect lasts 2 to 4 weeks after the inducer stops.
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Sources
- Australian Medicines HandbookAU/NZAustralian Medicines Handbook Pty Ltd, 2026
- Goodman & Gilman's The Pharmacological Basis of Therapeutics (14th ed.)McGraw Hill (Brunton and Knollmann, 2023), 2023
- Flockhart Table of cytochrome P450 drug interactionsIndiana University School of Medicine, Division of Clinical Pharmacology
- Drug development and drug interactions: table of substrates, inhibitors and inducersUS Food and Drug Administration
- Clinical Pharmacogenetics Implementation Consortium guideline for CYP2C19 genotype and clopidogrel therapy: 2022 updateClinical Pharmacology & Therapeutics (Lee et al., 2022;112:959-67), 2022
- Last updated
- 14 November 2026
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