## Drug of Choice for Urinary Retention **Key Point:** Bethanechol is the first-line cholinergic agent for post-operative and neurogenic urinary retention because it selectively stimulates M~3~ muscarinic receptors on bladder detrusor muscle, promoting micturition without significant cardiovascular effects. ### Mechanism of Action Bethanechol is a selective M~3~ agonist that: - Increases detrusor muscle contractility - Promotes bladder emptying - Has minimal nicotinic effects (does not cause fasciculations) - Does not cross the blood-brain barrier significantly ### Pharmacokinetic Advantage - **Oral bioavailability:** Better than other cholinergic agents due to resistance to acetylcholinesterase - **Onset:** 30–90 minutes (oral); 5–15 minutes (subcutaneous) - **Duration:** 1–2 hours (oral); 30–90 minutes (subcutaneous) ### Clinical Use in BPH with Retention | Feature | Bethanechol | Pilocarpine | Carbachol | |---------|-------------|------------|----------| | **Selectivity** | M~3~ selective | Non-selective M | Non-selective M + N | | **Bladder effect** | Strong detrusor contraction | Moderate | Strong but unpredictable | | **Systemic cholinergic effects** | Minimal | Marked (salivation, sweating) | Marked | | **Cardiovascular risk** | Low | High (bradycardia, hypotension) | High | | **Route** | Oral, SC preferred | Oral, topical | Oral, topical | | **DOC for urinary retention** | **Yes** | No | No | **High-Yield:** Bethanechol is the ONLY cholinergic agent recommended for urinary retention because it avoids the severe systemic side effects of non-selective agents. **Clinical Pearl:** Bethanechol is contraindicated in mechanical obstruction (e.g., urethral stricture) and should not be given IV or IM (risk of severe cholinergic crisis). **Tip:** Remember: **Bethanechol = Bladder** — the "B" mnemonic links it to bladder emptying.
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