H2 Antagonists

Recall the parietal cell and its three "go" signals for acid: histamine, gastrin, and acetylcholine, all converging on the H⁺/K⁺-ATPase pump. Histamine — released by neighboring ECL cells — is the most important amplifier: it binds H2 receptors on the parietal cell and drives up cyclic AMP (cAMP), which switches the pump into high gear. Blocking that H2 receptor is exactly what these drugs do.

An H2 antagonist competitively and reversibly blocks the H2 receptor, lowering cAMP and cutting acid secretion — particularly the basal and nighttime (nocturnal) acid, which is largely histamine-driven. Relief comes on relatively fast (within an hour). But notice the ceiling: because the drug blocks only the histamine arm, gastrin and acetylcholine can still drive the pump, so H2 blockers are less complete than PPIs — which disable the pump itself downstream of all three signals.

H2 blockers have a peculiar weakness: tolerance (tachyphylaxis) develops within about 1–2 weeks of regular use, so the same dose suppresses less acid over time. The reason is a feedback loop — lowering acid raises gastrin, which pushes ECL cells to release more histamine and up-regulates the parietal cell’s H2 machinery, partly overriding the blockade. This, plus their shorter duration, is why H2 blockers are excellent for on-demand or nighttime symptom relief but tend to lose ground to PPIs for healing erosive disease, where sustained, profound suppression is needed.

A crucial safety update shapes the class today: the original ranitidine (Zantac) was withdrawn from the market in 2020 because it could form NDMA, a probable human carcinogen, that increased over time and with heat. So the modern mainstays are famotidine (preferred — clean interaction profile) and, less often, cimetidine. The distinction matters because cimetidine is the problem child: it inhibits several CYP450 enzymes (raising levels of warfarin, phenytoin, theophylline, and more) and has antiandrogenic effects (gynecomastia, impotence). Famotidine and nizatidine do neither, which is why famotidine is generally chosen.

As a class, H2 blockers are remarkably well tolerated — mild headache, dizziness, or a change in bowel habit at most. The serious effects appear in two settings. The first is accumulation: famotidine is cleared largely by the kidneys, so in renal impairment or the elderly the drug builds up and can cross into the brain, causing confusion, delirium, and agitation. The fix is to reduce the dose in renal disease and watch mental status — the 2023 Beers Criteria flag these agents in older adults at risk of delirium.

The second setting is cimetidine-specific. Its CYP450 inhibition slows the metabolism of many narrow-margin drugs — warfarin (bleeding), phenytoin and theophylline (toxicity) — so it demands interaction screening or, better, substitution with famotidine. Its antiandrogenic action produces gynecomastia and impotence, dose-related and reversible. As with PPIs, very long-term acid suppression can modestly impair vitamin B12 absorption. Reassuringly, H2 blockers carry no boxed warning.