α-blockers compared — selectivity, uses and adverse effects
Verified · Oct 2026Which receptors an α-blocker reaches decides what it does: block α2 as well as α1 and norepinephrine spills over onto β receptors; spare α2 and reflex tachycardia is smaller; prefer the prostate’s α1A and orthostatic effects become relatively minimal. Labetalol and carvedilol add β blockade to α1 blockade.
Four ways to block an α receptor
α1 receptors couple to Gq: phospholipase C forms IP3 and DAG and intracellular calcium rises, so vascular smooth muscle contracts. They also set smooth-muscle tone in the prostate, prostatic capsule, prostatic urethra and bladder neck. α2 receptors on nerve endings couple to Gi and inhibit norepinephrine release when activated — a feedback brake.
Nonselective blockers (phentolamine, phenoxybenzamine) block α1 and α2: blocking α2 increases norepinephrine release, and the spillover onto β receptors causes tremulousness and tachycardia. α1-selective blockers (prazosin, doxazosin, terazosin) leave α2 intact and cause less reflex tachycardia. Uroselective agents aim at the prostate, where about 70% of α1 receptors are α1A: tamsulosin is selective for prostatic α1 receptors and silodosin binds α1A with high affinity. Labetalol combines selective α1 blockade with nonselective β blockade, and carvedilol, a nonselective β-blocker, blocks α1 in both of its enantiomers.
Comparison
Selectivity and what follows
| Drug | Receptors blocked | Binding | Heart rate | Main use |
|---|---|---|---|---|
| Phentolamine | α1 + α2 (nonselective) | Reversible; relatively short duration | Tachycardia: α2 blockade releases more norepinephrine onto β receptors; also direct positive chronotropic effects | Hypertensive episodes of pheochromocytoma; norepinephrine extravasation |
| Phenoxybenzamine | α1 + α2 (nonselective) | Irreversible, covalent; effects last days | Reflex (baroreceptor) tachycardia | Before pheochromocytoma surgery (started 10–14 days ahead) |
| Prazosin, doxazosin, terazosin | α1-selective | Competitive (prazosin, doxazosin) | Less reflex tachycardia than nonselective blockers (α2 left intact) | Hypertension (second-line); doxazosin and terazosin also BPH |
| Tamsulosin, silodosin, alfuzosin | Prostatic α1 (about 70% α1A); silodosin high α1A affinity | — | — | BPH — not indicated for hypertension |
| Labetalol | Selective α1 + nonselective β; α:β about 1:3 oral, 1:7 IV | Competitive | Blood pressure falls without reflex tachycardia | Severe hypertension (IV); hypertension in pregnancy |
| Carvedilol | Nonselective β (S(−) enantiomer) + α1 (both enantiomers) | — | — | Chronic HFrEF, hypertension, LV dysfunction after MI |
Metyrosine sits on the α-blocker page but blocks no receptor: it inhibits tyrosine hydroxylase, the rate-limiting step of catecholamine synthesis.
— = no statement for that cell on the class pages
Signature adverse effects
| Drug | Postural hypotension | Floppy iris (IFIS) | Ejaculation | Other signature effect |
|---|---|---|---|---|
| Phentolamine | Hypotension (most common toxicity of α-blockade) | — | — | MI and cerebrovascular events, usually with marked hypotension; epinephrine can cause a paradoxical fall in blood pressure |
| Phenoxybenzamine | Postural hypotension | — | Inhibited ejaculation | Epinephrine reversal — with α blocked, epinephrine’s net effect is vasodilation; nasal congestion, miosis |
| Prazosin, doxazosin, terazosin | First-dose syncope — risk greatest in the first 7 days, after a rapid dose increase or a restart | Yes (α1 on the iris dilator) | — | — |
| Tamsulosin, silodosin, alfuzosin | Relatively minimal orthostatic effects | Most often with tamsulosin; can occur after stopping | Retrograde ejaculation — silodosin’s most common adverse reaction | Strong CYP3A4 inhibitors contraindicated (alfuzosin, silodosin) or not combined (tamsulosin) |
| Labetalol | Blood pressure lowered more standing than supine | — | — | Bronchospasm risk from nonselective β blockade; paradoxical hypertension in a few patients with pheochromocytoma |
| Carvedilol | Blood pressure lowered more standing than supine | — | — | Bronchospasm risk from nonselective β blockade; more dizziness on up-titration in CYP2D6 poor metabolizers (2–3-fold R(+) carvedilol) |
— = no statement for that cell on the class pages
Choosing by situation
| Situation | Choice | Why |
|---|---|---|
| Hypertension and symptomatic BPH | Doxazosin or terazosin | Both are labelled for BPH and hypertension; α1 blockade relaxes prostatic and bladder-neck smooth muscle |
| BPH without hypertension | Tamsulosin, silodosin or alfuzosin | They target prostatic α1 with relatively minimal orthostatic effects; none is indicated for hypertension |
| Preparing a pheochromocytoma for surgery | Phenoxybenzamine for 10–14 days; a β-blocker only after α-blockade | β-blockade first leaves α stimulation unopposed — a hypertensive crisis; labetalol is not preferred because IV it favours β about 7:1 |
| Hypertensive surge in pheochromocytoma, or norepinephrine extravasation | Phentolamine | A reversible, short-acting α-blocker given IV or IM; it is indicated for hypertensive episodes during preparation and surgical excision, and for dermal necrosis after norepinephrine extravasation |
| Cataract surgery planned | Do not start tamsulosin before it; tell the surgeon about any α1-blocker | Floppy iris syndrome comes from long-term α1 blockade of the iris dilator and can occur after the drug is stopped |
| Chronic HFrEF | Carvedilol | It is indicated for chronic HFrEF therapy; its α1 blockade adds vasodilation to nonselective β blockade |
| Severe hypertension in pregnancy | IV labetalol | Labetalol is one of the most commonly used drugs for hypertension in pregnancy, and the injection is labelled for severe hypertension |
| Cocaine toxicity | No β-blocker without concurrent α-blockade | Unopposed α stimulation can cause severe hypertension and death |
High-yield
Practice questions
Q1A 72-year-old man with hypertension and benign prostatic hyperplasia takes his first terazosin capsule in the evening. That night he lost consciousness for a few seconds when he got up to urinate, and he recovered fully on lying flat.
What best explains the episode?
- A.β1 blockade slowing the heart
- B.Central α2 agonism reducing sympathetic outflow
- C.An excessive postural fall in blood pressure from α1 blockade, greatest with the first doses
- D.Covalent α-receptor binding that builds up over a week
- E.α2 blockade releasing norepinephrine onto β receptors
Show answer
C. An excessive postural fall in blood pressure from α1 blockade, greatest with the first doses — Syncope after the first dose of an α1-blocker is believed to be due to an excessive postural hypotensive effect: the risk is greatest in the first 7 days of treatment, and it recurs when therapy is interrupted and restarted or the dose is increased. Terazosin is a selective α1 antagonist, not a β1-blocker, and it leaves the α2 brake on norepinephrine release intact, so norepinephrine spillover onto β receptors is less than with nonselective α-blockers. Central α2 agonism is clonidine’s action, and covalent blockade that builds up over about a week belongs to phenoxybenzamine.
Q2During removal of a catecholamine-secreting tumor, a patient given IV phentolamine develops a heart rate of 140/min with tremulousness. A colleague notes that patients on prazosin rarely react this way.
What best explains the difference?
- A.Prazosin also blocks cardiac β1 receptors
- B.Phentolamine binds α1 receptors irreversibly
- C.Phentolamine is selective for prostatic α1A receptors
- D.Phentolamine inhibits tyrosine hydroxylase
- E.Phentolamine blocks presynaptic α2 receptors, so more norepinephrine reaches β receptors
Show answer
E. Phentolamine blocks presynaptic α2 receptors, so more norepinephrine reaches β receptors — Phentolamine is a nonselective α-blocker: blocking the α2 brake on nerve endings increases norepinephrine release, and the spillover stimulates β receptors, causing tachycardia and tremulousness; it also has direct, less marked positive chronotropic effects. Prazosin is α1-selective and leaves α2 intact, so reflex tachycardia is smaller. Phentolamine binds reversibly (phenoxybenzamine is the irreversible one), prostatic α1A selectivity belongs to uroselective agents such as tamsulosin and silodosin, and tyrosine hydroxylase is inhibited by metyrosine.
Q3A 45-year-old woman with paroxysmal headaches and palpitations has just been diagnosed with a pheochromocytoma. Before any α-blocker has been started, she is given IV labetalol for a blood pressure of 190/110 mm Hg, and her pressure climbs further.
Which property of labetalol best explains the rise?
- A.It stimulates α1 receptors directly
- B.Given IV it blocks β receptors far more than α receptors, leaving tumor catecholamines acting on α
- C.It binds α receptors irreversibly
- D.It has strong intrinsic sympathomimetic activity at β1
- E.It blocks α2 receptors on nerve endings
Show answer
B. Given IV it blocks β receptors far more than α receptors, leaving tumor catecholamines acting on α — Intravenous labetalol has an α:β ratio of about 1:7, so β blockade dominates; with β receptors blocked before adequate α-blockade, circulating catecholamines act unopposed at α receptors, and paradoxical hypertensive responses have been reported in a few patients with this tumor. Its α1 blockade is selective and competitive, not irreversible and not α2, and its β1 intrinsic sympathomimetic activity is minimal.
Q4A 60-year-old man with heart failure with reduced ejection fraction has his carvedilol increased and feels dizzy and light-headed on standing. Genotyping shows he is a CYP2D6 poor metabolizer.
What best explains his symptoms?
- A.S(−) carvedilol levels triple, deepening β blockade
- B.Carvedilol blocks α2 receptors on nerve endings
- C.Poor metabolizers clear carvedilol faster, so it is less effective
- D.Higher levels of R(+) carvedilol, which blocks α1 receptors and dilates vessels
- E.Carvedilol has intrinsic sympathomimetic activity
Show answer
D. Higher levels of R(+) carvedilol, which blocks α1 receptors and dilates vessels — CYP2D6 poor metabolizers have 2–3-fold higher R(+) carvedilol but only about 20–25% more S(−) carvedilol, and they have more dizziness during up-titration, presumably from the vasodilating effect of the α-blocking R(+) enantiomer. Carvedilol’s α blockade is at α1, it has no intrinsic sympathomimetic activity, and its α1 blockade lowers blood pressure more standing than supine.
Sources
- Adrenergic Drugs — α1 Gq, α2 Gi — StatPearls (NCBI)
- Alpha-Blockers — nonselective vs selective, uroselective, adverse-effect mechanisms, floppy iris — StatPearls (NCBI)
- Phenoxybenzamine — irreversible blockade — StatPearls (NCBI)
- Perioperative Management of Pheochromocytoma — α before β, labetalol — StatPearls (NCBI)
- Phentolamine Mesylate for Injection (Hikma) — FDA label — FDA / DailyMed
- Phenoxybenzamine Hydrochloride Capsules (Amneal) — FDA label — FDA / DailyMed
- Tamsulosin Hydrochloride Capsules (Zydus) — FDA label — FDA / DailyMed
- Uroxatral (alfuzosin) — FDA label — FDA / DailyMed
- Silodosin Capsules (Lupin) — FDA label — FDA / DailyMed
- Demser (metyrosine) — FDA label — FDA / DailyMed
- Prazosin — StatPearls (NCBI)
- Doxazosin — StatPearls (NCBI)
- Clonidine — StatPearls (NCBI)
- Prazosin HCl capsules (ANI) — FDA label — FDA / DailyMed
- CARDURA (doxazosin) — FDA label — FDA / DailyMed
- Terazosin capsules (Jubilant Cadista) — FDA label — FDA / DailyMed
- CATAPRES-TTS (clonidine transdermal) — FDA label — FDA / DailyMed
- Propranolol — β2 blockade, cocaine toxicity — StatPearls (NCBI)
- Labetalol — StatPearls (NCBI)
- Carvedilol — StatPearls (NCBI)
- COREG (carvedilol) tablets — FDA label — FDA / DailyMed
- Labetalol hydrochloride tablets (Zydus) — FDA label — FDA / DailyMed
- Labetalol hydrochloride injection (Hospira) — FDA label — FDA / DailyMed
- Beta Blockers — receptor subtypes, selective vs non-selective agents, α1 blockade, CNS effects — StatPearls (NCBI)
- Toprol-XL (metoprolol succinate) — clinical pharmacology, CYP2D6, overdosage (FDA label) — FDA / DailyMed
- Stable Angina — oxygen supply–demand mismatch — StatPearls (NCBI)
- Nadolol tablets (Amneal) — clinical pharmacology, renal excretion (FDA label) — FDA / DailyMed
- Bystolic (nebivolol) — selectivity by CYP2D6 phenotype, pharmacokinetics (FDA label) — FDA / DailyMed
- BREVIBLOC (esmolol) injection — FDA label — FDA / DailyMed
- Tenormin (atenolol) — clinical pharmacology, renal elimination (FDA label) — FDA / DailyMed
Each class page compared lists its full sources; this page reuses facts verified there.
By LearnDrugs
Educational summary for medical students. Always verify against current prescribing information and guidelines. Not medical advice.