Exam aid · Drugs and therapeutics

Enzyme Inducers & Inhibitors

Most interaction questions come down to one inducer or inhibitor and one narrow-therapeutic-index victim drug.

Tables to memorise

Cover the right-hand column and test yourself before checking.

Inducer	Notes
Rifampicin	Strongest common inducer; OCPs, warfarin, HIV drugs
Carbamazepine	Also induces its own metabolism
Phenytoin	Induces CYP3A4 and others
Phenobarbital	Classic broad inducer
St John's wort	Reduces ciclosporin, OCP and HIV drug levels
Smoking	Induces CYP1A2 (theophylline, clozapine)
Chronic alcohol	Induces CYP2E1 (paracetamol toxicity risk)
Griseofulvin	Reduces OCP efficacy

Inhibitor	Enzyme / key victim
Ketoconazole, itraconazole	CYP3A4: statins, ciclosporin
Clarithromycin, erythromycin	CYP3A4: simvastatin, colchicine
Ritonavir	CYP3A4 (used to boost other drugs)
Grapefruit juice	Intestinal CYP3A4: simvastatin, felodipine
Fluconazole	CYP2C9: warfarin, sulfonylureas
Amiodarone	CYP2C9 + P-gp: warfarin, digoxin
Fluoxetine, paroxetine	CYP2D6: tamoxifen, codeine
Omeprazole	CYP2C19: clopidogrel activation
Ciprofloxacin	CYP1A2: theophylline
Cimetidine	Several CYPs
Valproate	Glucuronidation: lamotrigine
Allopurinol	Xanthine oxidase: azathioprine, mercaptopurine

Drug	Monitor
Warfarin	INR
Digoxin	Level, potassium, renal function
Lithium	Level 12 h post-dose, renal and thyroid function
Phenytoin	Level (free if albumin low)
Theophylline	Level, smoking status
Aminoglycosides	Trough and peak levels, renal function
Ciclosporin, tacrolimus	Trough (whole blood for tacrolimus)
Carbamazepine	Level after autoinduction

Induction takes days to weeks (new enzyme must be made); inhibition is fast.
After stopping an inducer, levels of other drugs rise again over 1–2 weeks.
Rifampicin users need extra contraception during and for 4 weeks after treatment.
Stopping smoking can raise theophylline and clozapine to toxic levels.
Low-dose ritonavir is used deliberately as a pharmacokinetic booster.

CRAP GPS (inducers): Carbamazepine, Rifampicin, Alcohol (chronic), Phenytoin, Griseofulvin, Phenobarbital, Sulfonylureas/St John's wort.
SICKFACES.COM (inhibitors): Sodium valproate, Isoniazid, Cimetidine, Ketoconazole, Fluconazole, Alcohol (acute), Chloramphenicol, Erythromycin, Sulfonamides, Ciprofloxacin, Omeprazole, Metronidazole.

Attempt MCQs on these topics while the sheet is fresh. Anything you get wrong goes to your incorrect list for the next round.