# Alpha Blocker Drugs

> Alpha blockers — phenoxybenzamine, phentolamine, prazosin, tamsulosin — pheochromocytoma, BPH and first-dose hypotension in MBBS Pharmacology notes.

- Canonical URL: https://prepelephant.com/topics/mbbs/pharmacology/alpha-blockers
- Exam / course: MBBS · Subject: Pharmacology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Alpha Blocker Drugs", PrepElephant, https://prepelephant.com/topics/mbbs/pharmacology/alpha-blockers

## Direct answer

Alpha blockers inhibit adrenergic alpha receptors: non-selective agents (phenoxybenzamine, phentolamine) block alpha-1 and alpha-2 and are reserved for pheochromocytoma and catecholamine-excess crises, while selective alpha-1 blockers (prazosin, doxazosin, tamsulosin) lower blood pressure by relaxing arterioles and venules and relieve bladder-outlet obstruction in benign prostatic hyperplasia. Their signature adverse effects are first-dose hypotension and syncope with prazosin, and ejaculatory dysfunction with tamsulosin.

## What you must remember

- Phenoxybenzamine is an irreversible, non-competitive alpha blocker; phentolamine is reversible — both are non-selective.
- In pheochromocytoma, alpha blockade (phenoxybenzamine 10 mg twice daily, titrated 1–2 weeks before surgery) must precede beta blockade; giving a beta-blocker first causes unopposed alpha-mediated hypertensive crisis.
- Phentolamine IV rescues the tyramine–cheese reaction in patients on monoamine oxidase inhibitors and reverses noradrenaline extravasation necrosis (5–10 mg local infiltration).
- Prazosin causes first-dose syncope — start 0.5 mg at bedtime; it also remains a specific treatment for post-traumatic stress disorder nightmares.
- Tamsulosin is alpha-1A selective: it relaxes prostate and bladder-neck smooth muscle with little effect on blood pressure, ideal for BPH patients on other antihypertensives.
- Intraoperative floppy iris syndrome during cataract surgery is a recognised complication of tamsulosin — warn the ophthalmologist.
- Doxazosin is once-daily and the most lipid-soluble alpha-1 blocker; terazosin and doxazosin improve BPH symptoms and pressure together.
- Yohimbine (alpha-2 blocker) has no routine clinical role but is exam-famous as the pharmacological opposite of clonidine.

## How to work through a pheochromocytoma preparation

A 34-year-old with paroxysmal headaches, palpitations and sweating has a 24-hour urine metanephrine level five times normal and an adrenal mass on CT. Preoperative pharmacological preparation proceeds in a fixed sequence, and the reasoning matters more than the doses. Begin with phenoxybenzamine 10 mg twice daily, increasing every few days until blood pressure is controlled and nasal congestion appears — nasal stuffiness is the traditional clinical sign that alpha blockade is adequate, because it is the one alpha-mediated function you can see from the foot of the bed. Only after at least three to four days of alpha blockade add a beta-blocker (propranolol or metoprolol) for reflex tachycardia; if beta-blockade is given first, the unopposed alpha stimulation can precipitate a hypertensive emergency or pulmonary oedema. Restore volume — chronic vasoconstriction masks a contracted plasma volume that would otherwise crash the pressure the moment the tumour is clamped. Continue phenoxybenzamine until the day of surgery; intraoperative surges are handled by phentolamine boluses or nitroprusside, and hypotension after tumour removal by volume and noradrenaline. For inoperable tumours, phenoxybenzamine plus metyrosine is long-term therapy. This pathway explains why the non-selective, irreversible drug survives in modern practice at all.

## Where students slip

Candidates routinely write "alpha blockers are first-line antihypertensives" — they are not; thiazides, calcium channel blockers, ACE inhibitors and ARBs hold that position, with prazosin reserved for add-on use or specific situations because the first-dose phenomenon and reflex tachycardia limit it. The second recurring mistake is misassigning selectivity: tamsulosin is alpha-1A (prostate-selective, pressure-sparing), while prazosin, terazosin and doxazosin block alpha-1 broadly and lower pressure. A viva favourite is the postural hypotension shared by prazosin and the phenothiazines through different mechanisms — receptor blockade versus alpha-adrenergic suppression — and the examiner wants you to name first-dose timing as the practical discriminator. Finally, do not forget that clonidine withdrawal can mimic pheochromocytoma; the drug history distinguishes rebound sympathetic overactivity from a catecholamine-secreting tumour faster than any assay.

## Frequently asked questions

### Why must beta-blockade never precede alpha-blockade in pheochromocytoma?
Beta-blockade leaves alpha-mediated vasoconstriction unopposed, so blood pressure rises precipitously and can precipitate hypertensive encephalopathy or pulmonary oedema; always block alpha first.

### Which alpha blocker suits a BPH patient who is already on amlodipine?
Tamsulosin 0.4 mg at bedtime, because alpha-1A selectivity relaxes prostatic smooth muscle without adding meaningful antihypertensive effect or first-dose syncope.

### What is first-dose phenomenon and how is it prevented?
Prazosin's initial dose can cause sudden syncope within 30–90 minutes due to venodilatation with absent baroreflex compensation; start with 0.5 mg at bedtime and warn against standing.

### How is noradrenaline extravasation managed?
Stop the infusion and infiltrate the area with phentolamine 5–10 mg in saline to reverse local alpha-1 vasoconstriction, preventing tissue necrosis; topical nitroglycerine is an alternative.

### Which alpha blocker has a psychiatric indication?
Prazosin reduces trauma nightmares and improves sleep in post-traumatic stress disorder by damping central adrenergic signalling during rapid eye movement sleep.