CYP450 inhibitors
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What it means
Block CYP enzymes within hours → slower metabolism → HIGHER levels of substrates (toxic INR on warfarin, statin myopathy). Mnemonic SICKFACES.COM: Sodium valproate, Isoniazid, Cimetidine, Ketoconazole/fluconazole, Fluoxetine, Acute alcohol, Chloramphenicol, Erythromycin/clarithromycin, Sulfonamides, Ciprofloxacin, Omeprazole, Metronidazole + grapefruit juice.
Worked example
Clarithromycin + simvastatin → raised statin levels → rhabdomyolysis (hold the statin during the course).
In the exam
A patient on warfarin starts clarithromycin and the INR climbs to 8 because the antibiotic blocks the metabolism of warfarin.
What settles it
Inhibitors RAISE substrate levels fast (hours); inducers LOWER them slowly (days–weeks).
Classically confused with
CYP450 inducers
Source: BNF — Interactions