doxazosin
alpha-1 adrenergic blocker· Brands: Cardura, Carduran, Doxazosin MesylatePO
Mechanism of action
Doxazosin is a selective, competitive antagonist at alpha-1 adrenergic receptors. It blocks the action of circulating catecholamines that bind to alpha-1 receptors located in vascular smooth muscle, leading to vasodilation and a reduction in peripheral vascular resistance, which lowers blood pressure. In the genitourinary tract, alpha-1 receptor blockade relaxes smooth muscle in the prostate and bladder neck, improving urinary flow in men with benign prostatic hyperplasia.
Pharmacokinetics
Doxazosin is well absorbed after oral administration, with an absolute bioavailability of approximately 65-70%. Peak plasma concentrations occur within 2-3 hours for the immediate-release formulation, while the extended-release formulation has a delayed and prolonged absorption. The drug is extensively metabolised in the liver, primarily via CYP3A4, to inactive metabolites, and is excreted in the faeces and urine. The elimination half-life is about 22 hours, allowing once-daily dosing. Effects persist for up to 24 hours.
Indications & dosing
Hypertension
Initial dose 1 mg daily; titrate according to response. Max 16 mg/day.
- adult · PO: 1–16 mg once daily · Initial dose 1 mg daily; titrate according to response. Max 16 mg/day.
Benign prostatic hyperplasia (BPH)
Initial dose 1 mg daily; usual maintenance 1-8 mg daily. Max 8 mg/day.
- adult · PO: 1–8 mg once daily · Initial dose 1 mg daily; usual maintenance 1-8 mg daily. Max 8 mg/day.
Contraindications & precautions
- Hypersensitivity to doxazosin or other quinazoline derivatives · Includes history of angioedema with these agents.
- Severe hepatic impairment · Doxazosin is extensively metabolised in the liver; use with caution or avoid in severe impairment.
- Administer the first dose at bedtime to reduce the risk of first-dose hypotension.
- Advise patients to rise slowly from a sitting or lying position to minimise orthostatic hypotension.
- Use caution in patients with hepatic impairment; consider monitoring liver function.
- Titrate the dose gradually to avoid sudden reductions in blood pressure.
- In patients with benign prostatic hyperplasia, exclude prostate cancer before starting therapy.
Adverse effects
- Dizziness
- Headache
- Fatigue
- Orthostatic hypotension
- Syncopeserious
- Oedema
- Palpitations
- Nasal congestion
- Priapismserious
- Hepatotoxicityserious
Interactions
- amlodipinemoderateMonitor blood pressure, heart rate, and symptoms such as dizziness when initiating or increasing the dose of either drug. Advise patients to rise slowly. Consider dose adjustment if hypotension occurs.
Renal & hepatic adjustment
- Hepatic impairment (moderate): Use with caution. Consider dose reduction or slower titration, as plasma levels may be increased.
- Hepatic impairment (severe): Use with caution. Reduce dose; monitor for hypotension.
Monitoring, pregnancy & toxicity
- Blood pressure · Baseline, after each dose change, then periodically
- Heart rate · Baseline and during dose titration
- Signs of orthostatic hypotension (dizziness, lightheadedness) · Especially after initiation or dose increase
- Pregnancy: Not recommended in pregnancy unless the potential benefit justifies the risk; alternative antihypertensives are preferred.
- Toxicity: Hypotension and orthostatic dizziness · Supportive care: lie patient supine, IV fluids, vasopressors if needed, atropine if bradycardia.
- Toxicity: Shock and collapse · Treat as vasodilator overdose; monitor blood pressure and heart rate; use vasopressors.
- Toxicity: Drowsiness and lethargy · Supportive; ensure adequate hydration and avoid further dose.
Clinical notes
Doxazosin is used to treat hypertension and symptomatic benign prostatic hyperplasia. The main adverse effect is orthostatic hypotension, especially with the first dose, so it is recommended to start with a low dose and take it at bedtime. Caution is needed in elderly patients and those with hepatic impairment. Other side effects include dizziness, headache, fatigue, and peripheral oedema. Rarely, priapism and floppy iris syndrome during cataract surgery have been reported. Consult local guidelines for dosing and use in specific populations.
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BNF; local formulary