Introduction Other agents: cimetidine, famotidine, and nizatidine Mechanism indirectly inhibit proton pump by blocking stimulation via H2 receptors on parietal cells and ECL cells leads to ↓ H+ secretion by parietal cells no effect on gastic emptying time Clinical use peptic ulcer gastritis GERD Zollinger-Ellison syndrome Toxicity cimetidine potent inhibitor of P-450 antiandrogenic effects results in gynecomastia, impotence, and ↓ libido in males crosses blood-brain barrier results in confusion, dizziness, and headaches avoid in the elderly and mentally unstable crosses placenta cimetidine and rantidine ↓ creatinine clearance other H2 blockers have fewer side effects