prazosin
Alpha-1 adrenergic antagonist· Brands: Minipress, HypovasePO
Mechanism of action
Prazosin is a selective alpha-1 adrenergic receptor antagonist. It blocks postsynaptic alpha-1 receptors on vascular smooth muscle, causing vasodilation and a decrease in peripheral vascular resistance. This leads to a reduction in arterial blood pressure without significant reflex tachycardia, as it does not affect presynaptic alpha-2 receptors. It is used in the treatment of hypertension and also for symptoms of benign prostatic hyperplasia and nightmares associated with post-traumatic stress disorder.
Pharmacokinetics
The onset of action occurs within 1-2 hours, and the duration of the antihypertensive effect is at least 10 hours.
Indications & dosing
Hypertension
Starting dose 0.5 mg twice daily; titrate gradually to minimise first-dose hypotension.
- adult · PO: 0.5–5 mg twice daily · Starting dose 0.5 mg twice daily; titrate gradually to minimise first-dose hypotension.
Nightmares associated with post-traumatic stress disorder (PTSD)off-label
Off-label; start with 1 mg at bedtime and titrate slowly.
- adult · PO: 1–10 mg at bedtime · Off-label; start with 1 mg at bedtime and titrate slowly.
Raynaud's phenomenonoff-label
Off-label; usual range 1-5 mg daily.
- adult · PO: 1–5 mg once daily · Off-label; usual range 1-5 mg daily.
Benign prostatic hyperplasia (BPH)off-label
Off-label; alpha-1 blockers are used to relieve urinary symptoms.
- adult · PO: 1–5 mg twice daily · Off-label; alpha-1 blockers are used to relieve urinary symptoms.
Contraindications & precautions
- Hypersensitivity to prazosin or other quinazoline derivatives · Cross-sensitivity may occur.
- Severe organic cardiac disease, e.g., aortic stenosis · May precipitate severe hypotension.
- The first dose should be taken at bedtime, or while lying down, to reduce the risk of first-dose syncope.
- Patients should be advised to rise slowly from sitting or lying to standing to avoid orthostatic hypotension.
- Caution is required when prazosin is given concurrently with other antihypertensives, vasodilators, or PDE5 inhibitors due to additive hypotension.
- Caution in patients with severe hepatic impairment, as prazosin is extensively metabolised in the liver.
- Patients should be warned that prazosin may cause dizziness and somnolence, and to avoid driving or operating machinery until these effects are known.
Adverse effects
- Dizziness
- Headache
- Somnolence
- Nausea
- Asthenia
- Orthostatic hypotension
- Syncope (first-dose phenomenon)serious
- Priapismserious
- Palpitations
Renal & hepatic adjustment
- eGFR 0+: No dose adjustment required; use with caution in renal impairment.
- Hepatic impairment (mild): No specific adjustment; initiate at low dose and titrate cautiously.
- Hepatic impairment (moderate): Start with a lower initial dose (e.g., 0.5 mg PO) and titrate slowly; monitor for hypotension.
- Hepatic impairment (severe): Avoid use; if necessary, use lowest dose with close monitoring. (avoid)
Monitoring, pregnancy & toxicity
- Blood pressure · Before first dose, after each dose increase, and regularly during therapy
- Heart rate · After first dose and during titration; monitor for reflex tachycardia
- Signs of syncope · Within 60-90 minutes after initial dose and after dose increases
- Symptom response in PTSD · Periodically during therapy
- Pregnancy: Prazosin should be used during pregnancy only if the benefit justifies the potential risk. It is not a first-line antihypertensive in pregnancy. Neonatal bradycardia and hypotension have been reported.
- Toxicity: Hypotension, dizziness, syncope · Lie patient supine; administer IV fluids; if severe, use vasopressors (e.g., noradrenaline); monitor blood pressure and heart rate.
- Toxicity: Reflex tachycardia · Monitor; rarely requires treatment.
Clinical notes
Prazosin is used for hypertension and for the management of PTSD-related nightmares, especially in veterans. It may also be used for Raynaud's phenomenon and for urinary symptoms due to benign prostatic hyperplasia. The most significant adverse effect is first-dose hypotension, so the initial dose should be small and taken at bedtime. Postural hypotension may occur, and patients should be advised to rise slowly. Reflex tachycardia is less common than with non-selective alpha blockers. Prazosin is generally well tolerated; other side effects include dizziness, headache, and fatigue. It is contraindicated in patients with known hypersensitivity. Use with caution in patients with renal impairment or in those receiving other antihypertensive agents.
Log in to read the full prazosin monograph
A free account unlocks the complete drug reference, the interaction checker, prescription practice and every other learning tool.
BNF; local formulary