Cytochrome P450 2C9
Also known as: CYP2C9
Cytochrome P450 enzyme responsible for metabolising coumarin anticoagulants (warfarin, acenocoumarol), several NSAIDs, phenytoin and some sulfonylureas.
CYP2C9 inhibition (e.g. fluconazole, amiodarone, metronidazole, cotrimoxazole) is the central mechanism of interactions that raise INR with coumarins; induction (rifampicin) reduces the anticoagulant effect.
Substrates: warfarin, acenocoumarol, phenytoin, diclofenac, ibuprofen, celecoxib, glibenclamide, glimepiride, losartan.
Inhibitors: fluconazole, voriconazole, metronidazole, amiodarone, cotrimoxazole (sulfamethoxazole), cimetidine.
Inducers: rifampicin, phenobarbital, carbamazepine, St John's wort.
Essential in anticoagulant therapy: any drug that inhibits CYP2C9 can raise the INR within days and requires close monitoring and preventive dose reduction.
DailyMed/FDA (NIH/NLM) — approved labels (warfarin, fluconazole); EMC-UK (MHRA) — SmPC
