## Most Common Adverse Effect of Anticholinergics **Key Point:** Dry mouth (xerostomia) is the most frequent and earliest adverse effect encountered with anticholinergic drugs across all age groups and clinical settings. ### Mechanism Anticholinergic agents block muscarinic receptors on salivary glands, reducing saliva production. This occurs at therapeutic doses and is dose-dependent. ### Frequency and Clinical Significance | Adverse Effect | Frequency | Severity | Onset | |---|---|---|---| | Dry mouth | Very common (>50%) | Mild–moderate | Early (days) | | Urinary retention | Common (20–30%) | Moderate–severe | Variable | | Mydriasis/cycloplegia | Common (30–40%) | Mild–moderate | Early | | Constipation | Common (20–30%) | Mild–moderate | Gradual | | Bradycardia | Rare (<5%) | Mild | Uncommon | **High-Yield:** Dry mouth is so predictable that it serves as a clinical marker of adequate anticholinergic dosing. Tolerance does NOT develop to this effect, unlike accommodation to some other anticholinergic effects. ### Why Other Options Are Less Common - **Urinary retention:** Occurs in 20–30% of patients, particularly in older men with baseline urinary hesitancy; not as universal as dry mouth. - **Mydriasis and cycloplegia:** Occur in 30–40% but are often asymptomatic or mild; patients may not report them unless vision is blurred. - **Bradycardia:** Rare with anticholinergics; tachycardia is far more common due to central effects and reduced vagal tone. **Clinical Pearl:** In elderly patients, anticholinergics should be used cautiously due to cumulative anticholinergic burden and increased risk of cognitive impairment, despite dry mouth being the most common tolerable effect. [cite:KD Tripathi 8e Ch 6]
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