Cimetidine

How it works

It competitively blocks histamine H2 receptors on gastric parietal cells, reducing both basal and stimulated gastric acid secretion.

Prescribing in practice

• Cimetidine is a potent inhibitor of several cytochrome P450 enzymes and raises plasma concentrations of many drugs, including warfarin, phenytoin and theophylline, so co-prescribing requires careful review and dose adjustment.
• Doses should be reduced in renal impairment, and it can cause confusion in elderly or seriously ill patients.
• Acid suppression may mask the symptoms of gastric cancer, so alarm features should be investigated before attributing them to benign dyspepsia.

Monitoring

Routine laboratory monitoring is not required, but monitor the effects of interacting drugs, such as the INR in patients taking warfarin, and review renal function where relevant.

Counselling the patient

• Tell your prescriber about all your other medicines, as interactions are common.
• Report any persistent difficulty swallowing, weight loss or signs of bleeding.
• Let your team know if you feel confused, particularly if you are elderly or unwell.

Evidence & guidelines

H2-receptor antagonists are an established acid-suppressing option for peptic ulcer disease and reflux, though proton pump inhibitors are generally preferred first-line in NICE guidance.