Which H2-receptor antagonist is contraindicated in patients with severe renal impairment due to risk of CNS toxicity from accumulation of active metabolites?

Explanation

## H2-Receptor Antagonists and CNS Toxicity in Renal Impairment **Key Point:** Cimetidine is the H2-receptor antagonist most notorious for CNS toxicity (confusion, hallucinations, delirium) in patients with severe renal impairment, due to accumulation of the parent drug and its active metabolites when renal clearance is reduced. ### Renal Elimination Profile of H2 Blockers | H2 Blocker | Renal Excretion | Hepatic Metabolism | CNS Toxicity Risk in Renal Failure | | --- | --- | --- | --- | | **Cimetidine** | **60–70%** | **30–40%** | **Highest — active metabolite accumulation; CNS toxicity well-documented** | | Famotidine | 65–75% | Minimal | Requires dose reduction; CNS toxicity less prominent | | Ranitidine | 50–60% | 30–40% | Moderate; dose reduction needed | | Nizatidine | 35–40% | ~60% | Lowest risk; predominantly hepatic metabolism | **High-Yield:** Cimetidine is the H2 blocker with the most documented CNS adverse effects — including confusion, agitation, hallucinations, and delirium — particularly in elderly patients and those with renal impairment. This is attributed to accumulation of cimetidine and its sulfoxide metabolite when renal clearance is compromised (KD Tripathi, *Essentials of Medical Pharmacology*, 8th ed.; Harrison's *Principles of Internal Medicine*). ### CNS Toxicity Manifestations (Cimetidine) - Confusion and disorientation (especially in elderly) - Hallucinations and delirium - Agitation and restlessness - Tremor and myoclonus - Seizures (in severe cases) **Clinical Pearl:** Among all H2 blockers, cimetidine has the broadest adverse effect profile — it also inhibits CYP450 enzymes (causing multiple drug interactions), has antiandrogenic effects (gynecomastia, impotence), and crosses the blood-brain barrier more readily than other H2 blockers. These properties make it the most "contraindicated" H2 blocker in renal failure with CNS toxicity risk. **Why not Famotidine?** While famotidine does require dose reduction in severe renal impairment (CrCl < 30 mL/min), it is NOT the drug classically contraindicated for CNS toxicity from active metabolite accumulation. Famotidine is actually preferred over cimetidine in renal failure because it lacks the antiandrogenic and CYP-inhibitory effects of cimetidine. **Reference:** KD Tripathi, *Essentials of Medical Pharmacology*, 8th ed., Chapter on Drugs for Peptic Ulcer; Harrison's *Principles of Internal Medicine*, 21st ed.