Pharmacology · Class
Alpha blockers !
Medicines that block alpha adrenergic receptors, relaxing vessels and the prostate. Used for benign prostatic hyperplasia, hypertension and pheochromocytoma.
Updated 2026-11-01
What they are
Alpha-1 selective blockers (prazosin, doxazosin, terazosin, tamsulosin) relax arterioles and the smooth muscle of the prostate and bladder neck. The non-selective blockers phenoxybenzamine (irreversible) and phentolamine (reversible) block alpha-1 and alpha-2 and are used mainly for pheochromocytoma and, for phentolamine, extravasation of vasopressors.
Mechanism of action
Blocking alpha-1 receptors prevents norepinephrine from contracting vascular and prostatic smooth muscle, so resistance and urinary outflow obstruction fall. Tamsulosin prefers the alpha-1A subtype of the prostate, so it lowers blood pressure less.
Target: Alpha-1 adrenergic receptors (alpha-1A in the prostate); alpha-2 for the non-selective agents
Drugs in this class
- PrazosinA short-acting alpha-1 blocker for high blood pressure. Its first dose can cause a sudden faint, so treatment always…Alpha-1 adrenergic receptors
- TamsulosinAn alpha-1A blocker that relaxes the prostate and bladder neck so urine flows more easily in benign prostatic…Alpha-1A adrenergic receptor
- DoxazosinA long-acting alpha-1 blocker for benign prostatic hyperplasia and high blood pressure, taken once a day. The first…Alpha-1 adrenergic receptors
- PhenoxybenzamineAn irreversible alpha blocker used before surgery for pheochromocytoma, to control the hypertension and sweating…Alpha-1 and alpha-2 adrenergic receptors (covalent bond)
- PhentolamineA short-acting, reversible alpha blocker given by injection: the antidote when norepinephrine leaks from a vein, and a…Alpha-1 and alpha-2 adrenergic receptors
- TerazosinAn alpha-1 blocker for benign prostatic hyperplasia and high blood pressure. Like prazosin, it can cause fainting…Alpha-1 adrenergic receptors
Others in the class
- Alfuzosin
- Silodosin
- Urapidil
Test yourself
Think of the answer, then tap the question to see it.
What is the first-dose effect of alpha-1 blockers?
Marked orthostatic hypotension, sometimes with syncope, after the first dose or a dose increase.
Why does tamsulosin lower blood pressure less than prazosin?
It is relatively selective for alpha-1A receptors, which predominate in prostatic smooth muscle, over the vascular subtypes.
In pheochromocytoma, why is the alpha blocker started before the beta blocker?
Blocking beta-2 vasodilation first would leave the tumour's catecholamines acting unopposed on alpha receptors and could precipitate a hypertensive crisis.
How does phenoxybenzamine differ from phentolamine?
Phenoxybenzamine binds alpha receptors covalently (irreversible, long effect); phentolamine is a reversible, short-acting competitive blocker.
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: DailyMed - PRAZOSIN HYDROCHLORIDE capsule (DailyMed (U.S. National Library of Medicine), 2026); DailyMed - TAMSULOSIN HYDROCHLORIDE capsule (DailyMed (U.S. National Library of Medicine), 2026); DailyMed - PHENOXYBENZAMINE HYDROCHLORIDE capsule (DailyMed (U.S. National Library of Medicine), 2026); Farmacología básica y clínica (Katzung), cap. 10 (McGraw Hill, 2025).
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Representative model, for educational purposes only.