Cardura (Doxazosin) 1 mg / 2 mg / 4 mg / 8 mg Tablets: Prescription Selective Alpha-1A Adrenoceptor Blocker for Hypertension and BPH — Urinary Flow Relief, No OTC Alternative, Available in Generic

Information last reviewed: May 2026 — for educational purposes only.

Doxazosin is a selective alpha-1 adrenergic receptor blocker marketed under the brand name Cardura. By blocking alpha-1 receptors in arterial and venous walls, it causes vasodilation and lowers blood pressure. It is a prescription medication also indicated for benign prostatic hyperplasia (BPH), where it relaxes smooth muscle in the prostate and bladder neck to improve urinary flow.

Uses of Doxazosin

Doxazosin is approved for hypertension and for the signs and symptoms of BPH. In hypertension, it is generally used as add-on therapy rather than a first-line agent, particularly in men who also have BPH and thus benefit from its dual action. The extended-release form, Cardura XL, is specifically approved only for BPH and is not indicated for hypertension. Doxazosin is typically started at 1 mg once daily and titrated upward at two-week intervals to a maximum of 8 mg for hypertension.

Mechanism, Warnings, and Key Interactions

By selectively blocking alpha-1 receptors, doxazosin prevents norepinephrine from constricting blood vessels, leading to reduced peripheral vascular resistance and lower blood pressure. The most important safety concern is first-dose orthostatic hypotension — a significant drop in blood pressure upon standing, which can cause dizziness, lightheadedness, or syncope. To minimise this risk, the first dose and any dose escalation should be taken at bedtime. A second critical warning is intraoperative floppy iris syndrome (IFIS): doxazosin relaxes the iris dilator muscle, which can cause the iris to billow and prolapse during cataract surgery or other intraocular procedures. The ophthalmologist must be informed of doxazosin use before any eye surgery, even if the drug has been discontinued. The ALLHAT trial found higher rates of heart failure in the doxazosin arm compared to chlorthalidone, which has influenced guidelines to generally deprioritise alpha-blockers as primary antihypertensives. Common side effects include peripheral oedema, dizziness, and somnolence.

Available Strengths and Dosing

Doxazosin IR tablets are available in 1 mg, 2 mg, 4 mg, and 8 mg strengths and are taken once daily. Cardura XL extended-release tablets are available in 4 mg and 8 mg and are used solely for BPH. Titration for hypertension typically begins at 1 mg daily and may be increased to 2 mg, 4 mg, and up to 8 mg at two-week intervals, assessing tolerance before each increase. No significant pharmacokinetic drug interactions via CYP enzymes have been identified.

Pricing Overview

Generic doxazosin is widely available at low cost. Both IR and XL formulations have affordable generic alternatives. The once-daily dosing of the IR form makes it convenient and cost-effective for most patients. Contact Lucas Clinic for current pricing and dispensing options.

Frequently Asked Questions

Why is the first dose of doxazosin taken at bedtime?

Doxazosin can cause a dramatic drop in blood pressure with the initial dose — a phenomenon called first-dose orthostatic hypotension. This is most dangerous when standing, as it can lead to dizziness, fainting, and falls. By taking the first dose at bedtime, the patient is lying down during the period of maximum hypotensive effect, substantially reducing the risk of injury. The same principle applies when the dose is increased.

What is intraoperative floppy iris syndrome and why does it matter?

IFIS is a complication that can occur during cataract surgery in patients who have taken alpha-1 blockers such as doxazosin. The drug causes lasting relaxation of the iris dilator muscle, which can cause the iris to become flaccid, billow forward toward the cornea, and prolapse through surgical incisions even after the drug has been discontinued. This significantly increases the risk of surgical complications. Ophthalmologists and surgeons must know about any prior or current use of alpha-1 blockers to modify their surgical technique and use appropriate instruments.

What did the ALLHAT trial find about doxazosin?

The ALLHAT (Antihypertensive and Lipid-Lowering Treatment to Prevent Heart Attack Trial) found that patients randomised to doxazosin had a significantly higher rate of combined cardiovascular events — particularly heart failure — compared to those taking the thiazide-like diuretic chlorthalidone. The doxazosin arm was discontinued early as a result. This finding has led current hypertension guidelines to recommend against using alpha-blockers as monotherapy for hypertension unless there is a compelling co-indication such as BPH.

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