Pharmacology · Class
H2 receptor antagonists (H2 blockers) !
Acid reducers that block histamine at the parietal cell's H2 receptor. Weaker than PPIs; ranitidine was withdrawn over an NDMA impurity.
Updated 2026-11-01
What they are
Cimetidine and famotidine (and nizatidine) lower gastric acid in peptic ulcer, reflux and hypersecretory states. Proton pump inhibitors have replaced them in most situations because they suppress acid more completely.
Mechanism of action
Histamine released by enterochromaffin-like cells binds H2 receptors on the parietal cell; the receptor couples to Gs, raises cAMP and activates the proton pump. H2 blockers compete with histamine at the receptor. Basal and nocturnal acid fall most, and acid driven by food or gastrin also falls because histamine amplifies those signals. They do not touch the pump itself, so their effect is partial and wears off as the drug is cleared.
Target: Histamine H2 receptor
Drugs in this class
- FamotidineThe H2 blocker of choice now that ranitidine is gone: it lowers stomach acid with few interactions. Its dose is…Histamine H2 receptor
- CimetidineThe first H2 blocker. It lowers stomach acid but inhibits liver enzymes and has antiandrogen effects, so it causes…Histamine H2 receptor
Others in the class
- Nizatidine
- Ranitidine (withdrawn in the US and Colombia, 2020)
Test yourself
Think of the answer, then tap the question to see it.
Why was ranitidine withdrawn from the market in the US and Colombia in 2020?
Some products contained, or formed during storage, N-nitrosodimethylamine (NDMA), a probable human carcinogen.
Which H2 blocker causes the most drug interactions, and why?
Cimetidine, because it inhibits hepatic cytochrome P450 enzymes and slows the metabolism of warfarin, phenytoin, theophylline and lidocaine.
Through which second messenger does histamine stimulate the parietal cell?
cAMP: the H2 receptor couples to Gs, activates adenylyl cyclase and protein kinase A, which drives the proton pump.
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: Medications for the Treatment of Gastric Acidity - Gastroenterology - MSD Manual Professional Edition (MSD Manual Professional Edition, 2025); Alerta sanitaria sobre retiro del mercado Ranitidina | Invima (Instituto Nacional de Vigilancia de Medicamentos y Alimentos (INVIMA), 2020); Retiro del mercado Ranitidina solución inyectable | Invima (Instituto Nacional de Vigilancia de Medicamentos y Alimentos (INVIMA), 2020); eEML - Electronic Essential Medicines List (World Health Organization, 2025).
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Representative model, for educational purposes only.