Which adverse effect is characteristic of the first dose of non-selective alpha blockers like doxazosin and is most likely to occur within 30–90 minutes of administration?

Explanation

## First-Dose Phenomenon in Non-Selective Alpha Blockers **Key Point:** Non-selective alpha blockers cause a dramatic first-dose effect characterized by sudden vasodilation, severe orthostatic hypotension, and syncope within 30–90 minutes of the initial dose. ### Pathophysiology of First-Dose Syncope 1. **Acute α1B blockade in blood vessels** → Loss of sympathetic vasoconstrictor tone 2. **Sudden peripheral vasodilation** → Pooling of blood in capacitance vessels 3. **Reduced venous return and cardiac output** → Orthostatic hypotension 4. **Cerebral hypoperfusion** → Syncope or presyncope 5. **Tolerance develops** → Subsequent doses do not produce this effect (tachyphylaxis) ### Clinical Characteristics | Feature | Details | | --- | --- | | **Timing** | 30–90 minutes after first dose | | **Severity** | Can be profound; patients may lose consciousness | | **Mechanism** | Sudden loss of α1B-mediated vasoconstriction | | **Prevention** | Start with low dose, give at bedtime, warn patient to lie down if symptoms occur | | **Tolerance** | Develops within days; not seen with subsequent doses | **High-Yield:** The first-dose effect is a major reason to start non-selective alpha blockers at low doses and administer them at bedtime. Uroselective agents like tamsulosin do not produce this effect because they spare α1B receptors in blood vessels. **Clinical Pearl:** Patients should be counseled to take the first dose at bedtime and to lie down immediately if dizziness or syncope occurs. This prevents falls and injuries. **Warning:** Confusing the first-dose effect with hypertensive crisis is a common exam trap. The first-dose effect is characterized by hypotension, not hypertension.