Pharmacokinetics: Metabolism

Key points

• Phase I reactions (oxidation, reduction, hydrolysis) mainly via P450; phase II = conjugation (glucuronidation, sulphation, acetylation, methylation, glutathione).
• First-pass metabolism: oral drug absorbed → portal vein → liver before systemic circulation; reduces bioavailability.
• First-pass can be avoided by IV, sublingual or rectal administration.
• Drugs with extensive first-pass metabolism include GTN, lidocaine, propranolol, levodopa and morphine.

Decision points in this pathway

• High first-pass metabolism?

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