P450 Enzyme System

Key points

• Cytochrome P450: family of enzymes mainly in the liver, responsible for phase I metabolism; CYP3A4 and CYP2D6 are the most clinically important isoforms.
• Inducers (faster metabolism, ↓ drug effect): Rifampicin, Phenytoin, Carbamazepine, Phenobarbitone, Chronic alcohol, St John's Wort, Smoking (CYP1A2).
• Inhibitors (slower metabolism, ↑ drug effect/toxicity): Sodium valproate, Erythromycin/clarithromycin, Azole antifungals, Ciprofloxacin, Grapefruit juice, Acute alcohol, Omeprazole.
• Grapefruit juice inhibits intestinal CYP3A4 → ↑ levels of statins, CCBs, some benzodiazepines.
• Key interactions: P450 inhibitors ↑ warfarin effect; inducers ↓ warfarin effect and can cause contraceptive pill failure.

Decision points in this pathway

• Co-prescribed drug is an inhibitor or inducer?

The full step-by-step algorithm, including what to do at each branch, opens in the app for approved members.