## Distinguishing Prazosin from Doxazosin ### Pharmacokinetic Profile Comparison | Feature | Prazosin | Doxazosin | |---------|----------|----------| | **Half-life** | 2–3 hours (short) | 10–26 hours (long) | | **Dosing** | 2–3 times daily | Once daily | | **Onset of action** | Rapid (peak 1–3 hrs) | Slower, gradual | | **Reflex tachycardia** | Marked (due to rapid vasodilation) | Minimal (gradual onset) | | **First-dose effect** | Severe syncope risk | Mild | ### Key Mechanism Behind the Difference **Key Point:** Prazosin's rapid onset and short half-life cause sudden, steep drops in peripheral vascular resistance, triggering strong baroreceptor-mediated reflex sympathetic activation and tachycardia. Doxazosin's longer half-life and gradual onset allow the cardiovascular system to adapt, minimizing reflex tachycardia. **Clinical Pearl:** The "first-dose effect" (syncope, severe orthostatic hypotension) is characteristic of prazosin and occurs within 30–90 minutes of the first dose. This is why prazosin must be started at low doses (0.5 mg) at bedtime. Doxazosin, with its slow onset, rarely causes this dramatic response. **High-Yield:** Prazosin is preferred for hypertension in patients with concurrent benign prostatic hyperplasia (BPH) when reflex tachycardia is tolerable and dosing compliance is high. Doxazosin is preferred when once-daily dosing and minimal sympathetic activation are desired. ### Receptor Selectivity (Not the Distinguishing Feature) Both prazosin and doxazosin are **non-selective alpha-1 blockers** — they block alpha-1A, alpha-1B, and alpha-1D receptors equally. The distinction is **pharmacokinetic**, not pharmacodynamic. **Warning:** Do not confuse pharmacokinetic differences (half-life, onset) with receptor selectivity. Tamsulosin and alfuzosin are alpha-1A selective; prazosin and doxazosin are not.
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