Gastroenterology · USMLE Step 1 and Step 2

H2 Receptor Blockers song

H2 Receptor Blockers - Mechanism & Side Effects, as a song. Hear the short teaser here. The full song is on YouTube and Spotify.

H2 Receptor Blockers

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Key points in this song10

H2 receptor blockers (cimetidine, ranitidine, famotidine, nizatidine) competitively and reversibly block histamine H2 receptors on gastric parietal cells, reducing basal and meal-stimulated acid secretion.
Primary clinical uses include peptic ulcer disease, GERD, Zollinger-Ellison syndrome, and stress ulcer prophylaxis in ICU patients.
Cimetidine is the classic CYP450 inhibitor among H2 blockers, increasing levels of warfarin, phenytoin, theophylline, and other drugs metabolized by the liver.
Cimetidine also blocks androgen receptors, causing gynecomastia, impotence, and decreased libido in men, and galactorrhea in women.
H2 blockers decrease gastric acid by reducing cAMP-mediated proton pump activation, but are less potent than proton pump inhibitors for acid suppression.
Famotidine is the preferred H2 blocker in patients on multiple medications due to minimal CYP450 interaction and no antiandrogenic effects.
Long-term use of H2 blockers can lead to tolerance (tachyphylaxis) within days due to upregulation of histamine receptors and increased gastrin levels.
H2 blockers cross the blood-brain barrier and placenta; CNS side effects (confusion, hallucinations) are most common in elderly patients with renal impairment, particularly with cimetidine.
Unlike PPIs, H2 blockers do not require an acidic environment for activation and can be dosed on demand before meals; they are considered safe in pregnancy (Category B).
Ranitidine was withdrawn from the US market due to contamination with NDMA (N-nitrosodimethylamine), a probable human carcinogen; famotidine is now the preferred alternative.