Induce or Inhibit?

Which common drugs inhibit CYP450 enzymes and which induce them?

Here’s a couple of stories to help remember them – it’s by no means a comprehensive list, but should highlight the common drugs to remember.


Inducers – Mr Rifampicin

Mr Rifampicin tries St John’s Wort for the first time, which causes him to have two seizures.

He then rolls over and gets caught in barbed wire, where he proceeds to stay, and passes the time by having a long drinking session.

A wandering grizzly bear then mauls him to death.


Enzyme inducers:

  • Rifampicin
  • St John’s Wort
  • Phenytoin, Carbamazepine
  • Barbiturates
  • Chronic alcohol use
  • Griseofulvin
  • (also smoking, but via CYP1A2)


Inhibitors – Mrs Isoniazid

Mrs Isoniazid rolls in the New Year with a whopper of a booze binge.

This sets off her gout and causes her to vomit twice.

This then makes her feel depressed, despite the two antidepressants that she’s taking, and gives her palpitations.

She attempts to remedy the situation by having unprotected sex, which gives her HIV and a nasty infection.

She’s prescribed four separate antibiotics and two antifungals to clear everything up.


Enzyme Inhibitors:

  • Isoniazid
  • Acute alcohol use
  • Allopurinol
  • Cimetidine, Omeprazole
  • Fluoxetine, Sertraline
  • Amiodarone
  • Ritonavir
  • Antibiotics
    • Erythromycin
    • Metronidazole
    • Ciprofloxacin
    • Quinupristin
  • Antifungals
    • Ketoconazole
    • Fluconazole



Just remember than in questions they’ll often ask something like, “which of the following drugs would increase a warfarin patient’s INR” – an increased INR implies the enzymes have been inhibited so you need to think about the inhibitors…


Don’t get fooled!

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