Alpha-Adrenergic Antagonists (α-Blockers)
High-yield Verified · Oct 2026Prototype: tamsulosin
Alpha-Adrenergic Antagonists (α-Blockers) are a class of medications used for phentolamine to prevent or control hypertensive episodes in pheochromocytoma before and during surgery, and for the phentolamine blocking test.
Medical view → mechanisms, drug comparisons and Step 1 pharmacology
α-adrenergic receptor blockers for pheochromocytoma and benign prostatic hyperplasia — plus metyrosine, which is not a receptor blocker at all.
How it works in the body
The system involved, what goes wrong, and how the drug and body interact.
01 Two α receptors, three kinds of blocker
Most α1 receptors sit on vascular smooth muscle; when epinephrine or norepinephrine activates them, vessels constrict and blood pressure and peripheral resistance rise. α2 receptors on peripheral nerve endings do the opposite job: when activated they inhibit norepinephrine release, a feedback brake. α1 receptors are also abundant in the prostate, prostatic capsule, prostatic urethra and bladder neck, where they set smooth-muscle tone.
Nonselective blockers — phentolamine (reversible) and phenoxybenzamine (irreversible) — block both α1 and α2. They dilate vessels, but blocking α2 also releases more norepinephrine, which stimulates β receptors and causes tachycardia and tremulousness. They work best when sympathetic activity is high, as with the catecholamine surges of pheochromocytoma, and are meant for short-term use.
Selective α1 blockers end in -osin. The antihypertensive ones — prazosin, doxazosin and terazosin — are taught on the Cardiovascular › Other Antihypertensives page. This page covers the uroselective agents used for BPH: tamsulosin is selective for the α1 receptors of the prostate (about 70% of which are the α1A subtype), silodosin binds the α1A subtype with high affinity, and alfuzosin is a selective α1 antagonist at the prostate and bladder neck.
02 Pheochromocytoma: α first, β later — and metyrosine
A pheochromocytoma releases catecholamines that constrict vessels, so the patient is hypertensive yet volume-contracted. An α-blocker — most often phenoxybenzamine, which is long-acting — is started 10 to 14 days before surgery to control blood pressure and allow volume expansion. Phentolamine, short-acting and given IV or IM, prevents or controls hypertensive episodes before and during the operation.
A β-blocker is never started before the α-blockade is established: blocking β receptors first leaves catecholamines free to act on α receptors, and a hypertensive crisis can follow. Once α-blockade is in place, a β-blocker may be added if tachycardia is excessive.
Metyrosine is not an α-blocker. It inhibits tyrosine hydroxylase, the first and rate-limiting step of catecholamine synthesis (tyrosine to DOPA), so the tumor makes less norepinephrine and epinephrine. It is used to prepare patients for surgery, when surgery is not possible, and for malignant pheochromocytoma — but it does not eliminate the danger of hypertensive crises during tumor manipulation, when phentolamine may still be needed.
03 BPH: relaxing the dynamic obstruction
BPH obstructs the bladder outlet in two ways: a static part from prostate enlargement and a dynamic part from increased smooth-muscle tone in the prostate and bladder neck. That tone is driven by sympathetic stimulation of α1 receptors, so blocking them relaxes the smooth muscle, improves urine flow and reduces symptoms — usually within a few days. The uroselective agents are not indicated for hypertension.
Because BPH and prostate cancer cause many of the same symptoms and often coexist, prostate cancer is ruled out before an α-blocker is started. For cataract surgery, any of these drugs — most often tamsulosin — can cause intraoperative floppy iris syndrome, even after the drug has been stopped, so the ophthalmologist must be told.
Drug names
Indications
- Phentolamine to prevent or control hypertensive episodes in pheochromocytoma before and during surgery, and for the phentolamine blocking test
- Phentolamine injected locally to prevent or treat dermal necrosis and sloughing after norepinephrine extravasation
- Phenoxybenzamine to control episodes of hypertension and sweating in pheochromocytoma
- Metyrosine in pheochromocytoma to prepare for surgery, when surgery is contraindicated, and for chronic treatment of malignant pheochromocytoma
- Tamsulosin, alfuzosin and silodosin for the signs and symptoms of benign prostatic hyperplasia, not for hypertension
- Phentolamine as OraVerse to reverse lip and tongue anesthesia after a dental injection, and as Ryzumvi eye drops to reverse drug-induced pupil dilation
Mechanism of action
Block α-adrenergic receptors. Phentolamine (reversible) and phenoxybenzamine (irreversible, covalent) block both α1 and α2: vasodilation and lower vascular resistance, with reflex and α2-mediated tachycardia. Tamsulosin, silodosin (α1A-preferring) and alfuzosin block α1 receptors in the prostate, prostatic capsule, prostatic urethra and bladder neck, relaxing smooth muscle and improving urine flow. Metyrosine is not a receptor blocker: it inhibits tyrosine hydroxylase, the rate-limiting step of catecholamine synthesis.
Therapeutic effects — what you'll see working
Success is a controlled blood pressure with only mild orthostasis before pheochromocytoma surgery, or better urine flow and fewer urinary symptoms in BPH — without falls from orthostatic hypotension.
- Blood pressure control in pheochromocytoma
- Blocking α receptors relaxes arterioles and venous capacitance vessels and lowers vascular resistance, so catecholamine surges no longer drive the pressure up; metyrosine lowers catecholamine production itself.
- Better urine flow in BPH
- Blocking α1 receptors relaxes smooth muscle in the prostate and bladder neck, improving urine flow rate and reducing BPH symptoms.
- Rescue of skin after norepinephrine extravasation
- Phentolamine infiltrated into the area prevents or treats the dermal necrosis and sloughing that norepinephrine extravasation can cause.
Adverse effects
Most adverse effects are the α-blockade itself — orthostatic hypotension, reflex tachycardia, nasal stuffiness, ejaculation changes — plus the floppy iris of cataract surgery; metyrosine’s main effects are sedation and extrapyramidal signs.
Interactions
Contraindications
Labeled contraindications differ by drug: coronary artery disease or past myocardial infarction (phentolamine injection), any state where a fall in blood pressure is undesirable (phenoxybenzamine), moderate or severe hepatic impairment and potent CYP3A4 inhibitors (alfuzosin), severe renal or hepatic impairment and strong CYP3A4 inhibitors (silodosin), and hypersensitivity for all six. Tamsulosin and metyrosine list only hypersensitivity.
Nursing considerations
The RN-specific layer — each action paired with the reason it matters.
Common questions
What are Alpha-Adrenergic Antagonists (α-Blockers) used for?
How do Alpha-Adrenergic Antagonists (α-Blockers) work?
What are the serious side effects of Alpha-Adrenergic Antagonists (α-Blockers)?
Conditions treated
Alpha-1 blockers are first-line medical therapy and the fastest-acting option — they "minimize muscle tone in prostate stromal smooth muscle and bladder neck tissue," relaxing the dynamic component of the obstruction (the stroma is 80% of the hyperplastic volume, about half of it alpha-adrenergic smooth muscle). Expect 4 to 6 points of symptom-score improvement, with the effect maximizing in about 72 hours — which is why an alpha-blocker is started at the time of catheterization and the voiding trial is deferred at least 72 hours. Two disorder-side cautions: uroselective agents (tamsulosin, alfuzosin, silodosin) cause less orthostatic hypotension than terazosin or doxazosin, and dizziness, low blood pressure and ejaculatory dysfunction are common; and any alpha-blocker must be flagged before cataract or glaucoma surgery because of intraoperative floppy iris syndrome. The honest limitation: unlike 5-alpha-reductase inhibitors, alpha-blockers do not reduce the long-term risk of acute retention or the eventual need for surgery — they treat symptoms, not the disease.
Acutely, the alpha-blocker tamsulosin 0.4 mg orally daily is the most frequently used medical expulsive therapy (MET), relaxing distal ureteral smooth muscle so the stone can pass. The size band is larger than most people expect: pooled trial data show the benefit is concentrated in the larger ureteral stones — roughly 5 to 10 mm — with little gain for the smallest stones, which usually pass anyway, and the effect appears similar whatever the stone's position in the ureter. A stone that has not passed within 4 to 6 weeks, with or without this therapy, needs intervention. Silodosin has been shown superior to both tamsulosin and alfuzosin. Alpha-blockers are also used for the colic that follows lithotripsy and to ease ureteral stent discomfort.
Alpha-blockers are first-line and come first. Phenoxybenzamine "is the usual drug of choice; prazosin, terazosin, and doxazosin (selective alpha-1-antagonists) are alternative choices." Phenoxybenzamine is "an irreversible, nonspecific alpha-adrenergic blocker" with a "half-life of approximately 24 hours," and "Selective alpha-1-adrenergic blocking agents cause less reflex tachycardia." Started at diagnosis, for about 1 to 3 weeks before surgery. Expect orthostatic hypotension and reflex tachycardia, especially if volume is low; phenoxybenzamine’s listed adverse effects also include miosis, drowsiness, fatigue, inhibited ejaculation and gastrointestinal irritation. Phentolamine — "a short-acting non-selective alpha-1 and alpha-2 antagonist with a duration of action of 10 to 15 minutes" — treats acute episodes and intraoperative hypertension.
[Overflow] Alpha-blockers such as tamsulosin are listed for overflow alongside bethanechol (StatPearls).
See also
Turn UP “rest-and-digest.” Used for urinary retention, myasthenia, dry mouth and the eye; methacholine tests airway reactivity and physostigmine reverses anticholinergic poisoning. Overdose = cholinergic crisis (SLUDGE).
Block "rest-and-digest." Atropine treats bradycardia; glycopyrrolate dries secretions with fewer CNS effects; tropicamide and cyclopentolate drops dilate pupils; classic toxidrome — can’t see/pee/spit/poop.
Sources
- Phentolamine Mesylate for Injection (Hikma) — pheochromocytoma, extravasation, contraindications, warnings (FDA label) — FDA / DailyMed
- Phenoxybenzamine Hydrochloride Capsules (Amneal) — pheochromocytoma, epinephrine reversal, carcinogenicity (FDA label) — FDA / DailyMed
- Tamsulosin Hydrochloride Capsules (Zydus) — BPH, α1A selectivity, IFIS, priapism, CYP interactions (FDA label) — FDA / DailyMed
- Uroxatral (alfuzosin) — BPH, contraindications, postural hypotension, QT (FDA label) — FDA / DailyMed
- Silodosin Capsules (Lupin) — BPH, α1A binding, renal and hepatic contraindications, retrograde ejaculation (FDA label) — FDA / DailyMed
- Demser (metyrosine) — tyrosine hydroxylase inhibition, pheochromocytoma, sedation, crystalluria (FDA label) — FDA / DailyMed
- OraVerse (phentolamine) — reversal of dental soft-tissue anesthesia (FDA label) — FDA / DailyMed
- Ryzumvi (phentolamine ophthalmic) — reversal of pharmacologic mydriasis (FDA label) — FDA / DailyMed
- Alpha-Blockers — α1/α2 receptors, nonselective vs selective agents, adverse effects, floppy iris — StatPearls (NCBI)
- Phenoxybenzamine — irreversible covalent α-blockade, reflex tachycardia — StatPearls (NCBI)
- Perioperative Management of Pheochromocytoma — α before β, metyrosine, volume — StatPearls (NCBI)
Reviewed by Hae Suk Lee, RN
Educational summary for nursing students. Always verify against current prescribing information and your institution's protocols before administering. Not medical advice.