## Pharmacokinetic Distinction Between Prazosin and Doxazosin **Key Point:** The most clinically relevant discriminator between prazosin and doxazosin is their **half-life and dosing frequency**, which directly impacts patient compliance and clinical utility. ### Comparison Table | Feature | Prazosin | Doxazosin | |---------|----------|----------| | **Half-life** | 2–3 hours | 9–12 hours | | **Dosing frequency** | Three times daily (TID) | Once daily (OD) | | **Selectivity** | Selective α₁-blocker | Selective α₁-blocker | | **Reflex tachycardia** | Present | Present | | **Clinical uses** | HTN, PTSD | HTN, BPH | **High-Yield:** Both prazosin and doxazosin are **selective α₁-blockers** — they do NOT differ in selectivity. Both cause reflex tachycardia due to peripheral vasodilation and baroreceptor-mediated sympathetic activation. Both are used for hypertension; doxazosin has the additional advantage of efficacy in BPH due to its longer half-life allowing better urethral smooth muscle relaxation. ### Clinical Pearl Prazosin's shorter half-life necessitates **three-times-daily dosing**, which reduces compliance. Doxazosin's longer half-life permits **once-daily dosing**, making it more practical for long-term management. This pharmacokinetic difference is the primary reason doxazosin has largely replaced prazosin in modern clinical practice for both hypertension and BPH. ### Mechanism of Reflex Tachycardia (Both Drugs) Alpha-1 blockade → peripheral vasodilation → ↓ BP → baroreceptor reflex → ↑ sympathetic outflow → tachycardia. Both prazosin and doxazosin exhibit this effect equally; it is NOT a discriminator. [cite:KD Tripathi 8e Ch 12]
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