Prazosin !
A short-acting alpha-1 blocker for high blood pressure. Its first dose can cause a sudden faint, so treatment always starts at the lowest strength.
Updated 2026-11-01
No structure of prazosin bound to its receptor is available; the figure shows doxazosin, a quinazoline blocker of the same family, in the binding site of the human alpha-1A adrenergic receptor.
Mechanism of action
Prazosin is a quinazoline that competitively and reversibly blocks alpha-1 receptors on arterioles and veins, so peripheral resistance and venous return fall. Because it spares presynaptic alpha-2 receptors, reflex tachycardia is milder than with non-selective blockers. It also relaxes the smooth muscle of the prostate and bladder neck.
- Prazosin blocks alpha-1 receptors on arterioles and veins
- Norepinephrine can no longer constrict the vessels
- Resistance and blood pressure fall; prostatic muscle relaxes
Pharmacokinetics
- Absorption
- Oral; peak levels in about three hours (label)
- Distribution
- Highly bound to plasma proteins
- Metabolism
- Extensive metabolism in the liver
- Half-life
- About 2 to 3 hours (label), so it is taken more than once a day
Indications
- High blood pressure Arterial hypertension →
- Other uses: benign prostatic hyperplasia Benign prostatic hyperplasia →
- Other uses: nightmares and sleep problems in post-traumatic stress disorder, Raynaud phenomenon
Adverse effects
Common
- Dizziness and lightheadedness
- Headache and drowsiness
- Tiredness, weakness and lack of energy
- Palpitations
- Nausea, dry mouth and nasal congestion
Contraindications
- Known sensitivity to quinazolines or prazosin
Key interactions
- PDE-5 inhibitors (for example sildenafil)
- Additive fall in blood pressure and symptomatic hypotension
- Other antihypertensives and diuretics
- Additive hypotension
What is monitored
- Blood pressure lying and standing, especially after the first dose or a dose increase
- Dizziness and falls
Nursing care
- Before the first dose: teach that dizziness or fainting can happen in the first hours; treatment starts at the lowest strength.
- Avoid driving or hazardous tasks after the first dose and after a dose increase.
- Rise slowly from lying or sitting; sit or lie down if dizzy.
- Before cataract surgery, tell the eye surgeon about alpha blocker use.
- An erection lasting several hours is an emergency.
Pregnancy and breastfeeding
Human data are limited; it has been used, alone or with other drugs, in severe hypertension of pregnancy, and animal studies showed no malformations.
Test yourself
Think of the answer, then tap the question to see it.
Why does prazosin treatment always start at the lowest strength?
Because of the first-dose effect: marked orthostatic hypotension, sometimes with syncope, in the first hours after the first dose.
Why does prazosin cause less reflex tachycardia than phentolamine?
It is selective for alpha-1 receptors and spares presynaptic alpha-2 receptors, so norepinephrine release is not increased.
Which drug combination with prazosin can cause symptomatic hypotension?
PDE-5 inhibitors such as sildenafil, because both lower blood pressure.
Which psychiatric use does prazosin have outside its labelled indication?
Nightmares and sleep problems linked to post-traumatic stress disorder.
Related drugs
Related conditions
← Back to the class: Alpha blockers
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: DailyMed - PRAZOSIN HYDROCHLORIDE capsule (DailyMed (U.S. National Library of Medicine), 2026); Prazosin: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2025); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026); Farmacología básica y clínica (Katzung), cap. 10 (McGraw Hill, 2025).
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Representative model, for educational purposes only.