CYP450 Enzymes and Drug Interactions
The cytochrome P450 enzyme system metabolises most drugs. Knowing the major enzyme families and their common inhibitors and inducers is one of the highest-yield topics for pharmacists.
The key enzyme families
CYP3A4 metabolises the largest share of drugs — statins, many antibiotics, calcium-channel blockers. CYP2D6 handles many psychiatric drugs. CYP2C9 handles warfarin and NSAIDs. When a drug is metabolised by one of these, adding an inhibitor or inducer of that same enzyme creates an interaction.
You don't need to memorise every substrate — learn the major inhibitors and inducers, and check any new drug against the patient's list.
Inhibitors and inducers
Inhibitors slow the enzyme, raising levels of drugs metabolised by it. Strong CYP3A4 inhibitors include clarithromycin, ketoconazole, and grapefruit juice. Inducers speed the enzyme, lowering levels — rifampicin, carbamazepine, and St John's Wort are classic inducers.
The clinical rule: inhibitor added = higher levels of the substrate (reduce dose); inducer added = lower levels (may need to increase dose). When the interacting drug is stopped, the effect reverses — so review doses both when adding and when withdrawing.
Practical example
St John's Wort and the Pill
A patient on oral contraceptives starts St John's Wort for mood. St John's Wort induces CYP3A4, speeding the metabolism of the contraceptive hormones and reducing their effectiveness. The patient may experience contraceptive failure. The pharmacist recognises the induction, advises against the combination, and suggests a non-hormonal backup or a different mood approach through the prescriber.
Key takeaways
- CYP3A4, CYP2D6, and CYP2C9 are the most clinically important families.
- Inhibitors raise substrate levels; inducers lower them.
- Common inducers: rifampicin, carbamazepine, St John's Wort.
- Review doses both when adding AND when withdrawing an interacting drug.
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