Cimetidine
- Class: H2 blocker
- Used for: heartburn
- Say it: sye-MET-ih-deen
- Forms: tablet
CIMETIDINE is the MEDDLER. It calms the acid — then interferes with half the other drugs in the cupboard.
It inhibits several CYP enzymes, so warfarin, phenytoin and theophylline all climb. It is also anti-androgenic. Famotidine does the same acid job and meddles with nothing.
All H2 blockers reduce acid by blocking histamine at the parietal cell. Cimetidine alone is a potent inhibitor of several CYP enzymes and has anti-androgen effects — which is why ranitidine and famotidine replaced it.
Side effects
Common: headache, dizziness, breast enlargement
Serious: confusion in older adults
Typical dose
400 mg twice daily, or 800 mg at night
The whole daily dose can be given at night, which suits ulcer healing. Reduce it in renal impairment.
Check yourself
Why is famotidine generally preferred over cimetidine?
- Cimetidine inhibits CYP enzymes and has anti-androgen effects
- Cimetidine does not block H2 receptors effectively
- Famotidine also blocks the proton pump
- Cimetidine must be given intravenously
Show answer
A. Cimetidine inhibits CYP enzymes and has anti-androgen effects
The acid-blocking action is much the same across the class. Cimetidine's problem is everything else it does — raising the levels of warfarin, phenytoin and theophylline, and causing gynaecomastia.
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