Class: H2 Receptor Antagonist
Mech.: Competitive inhib. of the histamine H2 receptor, but not of the H1 receptor → inhib. of fasting and stim. acid secretion. No disruption of circadian rhythm of acid secretion. Inhib. of 80-90% of gastrin and
vagal-stim. acid secretion.
Absorption: Oral → rapid, good absorption. Single nighttime dose.
Dist.:
Metab.: Partial hepatic metab.
Excretion, t_: Excreted intact in urine. 1.5-3 hr.
Toxicity/S.E.s: Uncommon. Diarrhea, headaches, myalgias, skin rashes. Inhib. cyt. P-450 (less than cimetidine) → dose adjustment with phenytoin, warfarin, & theophylline. Occasional cardiac arrhythmias.
Utility: PUD, Zollinger-Ellison synd., acute stress ulcers, GERD
Special Features: Potency—famotidine > ranitidine/nizatidine > cimetidine.
Tachyphylaxis—50% less effective after 6 months. Rebound
hypersecretion 2° to receptor upregulation and inhib. of ATPase recycling.
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