Prazosin
Pronunciation: PRAY-zoh-sin.
- Generic name
- prazosin hydrochloride
- Drug class
- Selective alpha-1 adrenergic receptor blocker (alpha-blocker)
- Form
- Capsule or tablet, typically 0.5 mg/1 mg, 2 mg, and 5 mg strengths depending on market
- Route
- Oral
- Legal status
- Prescription-only
Availability and supply rules vary by country
Trade names by country: United States · United Kingdom · Canada · Australia · New Zealand
Australia: Minipress (5), APO-PRAZOSIN (generic) (5) · Full directory
What prazosin hydrochloride is and how it works
Prazosin is a prescription medicine that relaxes blood vessels by blocking alpha-1 adrenergic receptors, lowering blood pressure and, in some markets, also relaxing muscle in the prostate and bladder neck or improving blood flow in Raynaud's phenomenon. (1) (2)
The most important thing to know about prazosin is the 'first-dose' effect: a small number of people feel very dizzy or faint, or actually faint, within about 30 to 90 minutes of their very first dose (or after a dose increase). This is why treatment always starts at the lowest possible dose, taken at bedtime, and is increased slowly and only under your prescriber's guidance.
Mechanism of action: how prazosin hydrochloride works
Prazosin is a selective, competitive antagonist at post-synaptic alpha-1 adrenergic receptors, producing vasodilation of both arterioles and veins and a reduction in total peripheral resistance, without the reflex tachycardia typically seen with direct vasodilators. (1) (2)
What prazosin hydrochloride is used for
Hypertension (high blood pressure)
Used, alone or combined with a diuretic or other antihypertensive medicine, to treat hypertension. This is the on-label use in every market reviewed. (1) (2) (4)
Benign prostatic hyperplasia (BPH) symptoms
In the UK and Australia, also on-label for relieving urinary symptoms of benign prostatic hyperplasia. This is not an on-label use of prazosin in the United States. (2) (5)
Raynaud's phenomenon and Raynaud's disease
In the UK and Australia, also on-label for the symptomatic treatment of Raynaud's phenomenon and Raynaud's disease. (2) (5)
Congestive heart failure resistant to conventional therapy
In the UK specifically, also on-label for congestive heart failure that has not responded adequately to conventional treatment. (2)
Before taking prazosin hydrochloride
Do not take / not appropriate for
- Do not take prazosin if you have a known allergy to it or to other quinazoline-related medicines. (all countries) (1) (2)
- Prazosin is not recommended if you have a history of micturition syncope (fainting related to urination). (United Kingdom, European Union) (2)
- Do not take the 2 mg or 5 mg strength as your first dose -- treatment must always start on the lowest available strength. (all countries) (1)
Tell your clinician about
- Whether you take a PDE-5 inhibitor (such as sildenafil, tadalafil, or vardenafil) for erectile dysfunction or another condition, since combining these with prazosin can cause symptomatic low blood pressure.
- Whether you are due to have cataract surgery, given the risk of intraoperative floppy iris syndrome in people taking (or who have previously taken) an alpha-blocker.
- Any history of fainting, dizziness on standing, or heart rhythm problems.
- Whether you are pregnant, plan to become pregnant, or are breastfeeding.
- All other medicines you take, especially other blood-pressure-lowering medicines.
How to take prazosin hydrochloride
- Take your very first dose, and any increased dose, at bedtime if possible, and be prepared to lie down if you feel dizzy or faint -- this is a well-recognised, usually one-off effect.
- Avoid driving or hazardous tasks for 24 hours after your first dose or after any dose increase.
- Do not increase your own dose -- dose increases are made gradually and only under your prescriber's direction.
- Rise slowly from lying or sitting positions, especially early in treatment.
Procedures and treatment changes
Tell the treating team before surgery, procedures, or any medicine change. Do not alter treatment without the responsible clinician. (1) (2) (3) (4) (5) (6)
Missed dose
If you miss a dose, contact your prescriber or pharmacist for guidance, particularly if you have missed several days of treatment -- restarting after a gap can carry a similar 'first-dose' risk to starting treatment for the first time.
Overdose
Seek emergency medical help if an overdose is suspected. Signs can include severe dizziness, fainting, and very low blood pressure. Treatment involves lying the person down with legs raised and supportive care in hospital; prazosin is not effectively removed by dialysis because it is highly protein-bound. (2)
Side effects
Side effects and their urgency vary; report severe, persistent, or concerning symptoms.
Dizziness (around 10%).
What to notice: Dizziness (around 10%).
What to do: Discuss persistent or troublesome symptoms with the treating clinician or pharmacist. (1)
Headache (around 8%).
What to notice: Headache (around 8%).
What to do: Discuss persistent or troublesome symptoms with the treating clinician or pharmacist. (1)
Drowsiness (around 8%).
What to notice: Drowsiness (around 8%).
What to do: Discuss persistent or troublesome symptoms with the treating clinician or pharmacist. (1)
Lack of energy and general weakness.
What to notice: Lack of energy and general weakness.
What to do: Discuss persistent or troublesome symptoms with the treating clinician or pharmacist. (1)
Palpitations and nausea.
What to notice: Palpitations and nausea.
What to do: Discuss persistent or troublesome symptoms with the treating clinician or pharmacist. (1)
Nasal congestion, blurred vision, and fluid retention (oedema).
What to notice: Nasal congestion, blurred vision, and fluid retention (oedema).
What to do: Discuss persistent or troublesome symptoms with the treating clinician or pharmacist. (2)
Serious warnings
First-dose syncope (fainting) or severe dizziness — high caution
Symptoms: Sudden severe dizziness, weakness, or fainting, usually within 30 to 90 minutes of the first dose or a dose increase; occasionally preceded by a fast heartbeat.
Action: If you feel faint or dizzy, lie down immediately with your legs raised until it passes. This effect occurs in around 1% of people starting on 2 mg or more, is minimised by always starting on the lowest dose, and usually does not recur once your body adjusts. Tell your prescriber if it happens. (1) (2)
Priapism (prolonged erection) — emergency
Symptoms: An erection lasting more than 4 hours, whether or not it is painful.
Action: Seek emergency medical attention immediately -- prolonged erections can cause permanent tissue damage if not treated promptly. (2) (1)
Eye complications during cataract surgery — high caution
Symptoms: Not a symptom you would notice beforehand -- this is a surgical risk (intraoperative floppy iris syndrome).
Action: Tell your eye surgeon that you are taking, or have previously taken, prazosin or any other alpha-blocker before any cataract surgery, so they can plan accordingly. (1) (2)
Signs of a serious allergic reaction — emergency
Symptoms: Swelling of the face, lips, tongue, or throat; difficulty breathing; widespread rash or hives.
Action: Seek emergency medical help immediately. (2)
Interactions
- PDE-5 inhibitors (sildenafil, tadalafil, vardenafil) can cause symptomatic low blood pressure when combined with prazosin -- your prescriber will usually make sure you are stable on prazosin first and start the PDE-5 inhibitor at its lowest dose. (1) (2)
- Other blood pressure medicines, including diuretics and beta-blockers (such as propranolol), can add to prazosin's blood-pressure-lowering effect and increase dizziness/fainting risk. (1)
- Alcohol can increase dizziness and the risk of fainting, especially early in treatment. (2)
Pregnancy, breastfeeding, kidney/liver function, age
Pregnancy
US labelling places prazosin in Pregnancy Category C: no adequate, well-controlled human studies exist, so it should be used in pregnancy only if the potential benefit justifies the potential risk to the fetus. (1) (2) (3) (4) (5) (6)
Breastfeeding
Prazosin is excreted in small amounts in human breast milk; caution is advised, and use during breastfeeding should be discussed with your prescriber. (1) (2) (3) (4) (5) (6)
Children
Specific paediatric dosing/safety data were not found in the sources reviewed for this dossier; any use in children would be under specialist care. (1) (2) (3) (4) (5) (6)
Elderly
No specific age-based dose adjustment was identified beyond the general first-dose-effect caution that applies to everyone; older adults may be more susceptible to postural hypotension and should be monitored accordingly. (1) (2) (3) (4) (5) (6)
Hepatic Impairment
Specific hepatic dose-adjustment data were not found in the sources reviewed for this dossier; discuss with your prescriber if you have liver problems. (1) (2) (3) (4) (5) (6)
Renal Impairment
Specific renal dose-adjustment data were not found in the sources reviewed for this dossier; discuss with your prescriber if you have kidney problems. (1) (2) (3) (4) (5) (6)
Monitoring
Before starting
- Baseline blood pressure, including a check for postural (standing) blood pressure drop
- Review of other medicines, especially PDE-5 inhibitors, diuretics, and other antihypertensives
- History of fainting or micturition syncope
During treatment
- Close monitoring around the first dose and any dose increase, given the well-documented first-dose syncope risk.
- Ongoing blood pressure checks as your dose is titrated.
- Review for priapism or persistent dizziness at follow-up visits.
Storage
Store at room temperature, in the original packaging, out of reach and sight of children.
Study the drug class: Alpha blockers
Professional information
Clinician-facing context only, sourced from the current US DailyMed prazosin label, the UK Hypovase SmPC, and the Health Canada Minipress product monograph; verify against the current full label before prescribing.
United States
Indication: Hypertension (only on-label indication in the US)
Dose summary: Start 1 mg 2-3 times daily; titrate slowly to usual maintenance 6-15 mg/day (max up to 40 mg/day in divided doses in some patients); never start on 2 mg or 5 mg.
Renal context: No specific renal dose-adjustment data identified in the sources reviewed. (1)
United Kingdom
Indication: Hypertension, resistant heart failure, Raynaud's phenomenon/disease, BPH symptoms
Dose summary: Hypertension: start 500 mcg 2-3 times daily, titrate over 1-2 weeks to max 20 mg/day. Heart failure: start 500 mcg 2-4 times daily, titrate to 4-20 mg/day. Raynaud's: start 500 mcg twice daily, maintain 1-2 mg twice daily. BPH: start 500 mcg twice daily, maintain 2 mg twice daily.
Renal context: No specific renal dose-adjustment data identified in the sources reviewed. (2)
Hepatic guidance
Specific hepatic-impairment dosing guidance was not identified in the sources reviewed for this dossier.
Overdose note
Overdose produces hypotension; manage with recumbent positioning, volume expansion, and vasopressors as needed. Prazosin is highly protein-bound and not effectively removed by dialysis. (1) (2)
Frequently asked questions
Why do I have to start on such a low dose of prazosin?
Because of the 'first-dose' effect: about 1% of people starting on 2 mg or more experience sudden severe dizziness or fainting within 30 to 90 minutes of their first dose. Starting on the lowest strength and increasing slowly greatly reduces this risk. (1)
Is prazosin used for anything other than blood pressure?
In the United States, prazosin is only formally approved for hypertension. In the United Kingdom and Australia, it is also approved for benign prostatic hyperplasia symptoms and Raynaud's phenomenon/disease, and in the UK also for heart failure resistant to standard treatment. Always check which use applies to your prescription and country. (2) (5)
Is prazosin available in the US, UK, Europe, Canada, Australia, and New Zealand?
Yes. It is confirmed authorised in all of these regions -- as Minipress (US, Canada, Australia, and Minipress Laphal in France), Hypovase (UK), Alpress (a prolonged-release form in France), and generics such as APO-PRAZOSIN (New Zealand, Australia). (1) (2) (3) (4) (5) (6)
What is the mechanism of action of prazosin hydrochloride?
Prazosin is a selective, competitive antagonist at post-synaptic alpha-1 adrenergic receptors, producing vasodilation of both arterioles and veins and a reduction in total peripheral resistance, without the reflex tachycardia typically seen with direct vasodilators. (1) (2)
References
- PRAZOSIN HYDROCHLORIDE capsule -- full prescribing information (generic equivalent of Minipress)U.S. Food and Drug Administration / DailyMed (NIH) · United States · accessed 2026-07-30 · source 1
- Hypovase Tablets 0.5mg -- Summary of Product Characteristicselectronic medicines compendium (emc), United Kingdom -- Pfizer Limited · United Kingdom · accessed 2026-07-30 · source 2
- ALPRESS (prolonged-release prazosin) and MINIPRESS (immediate-release prazosin) -- HAS assessment summariesHaute Autorite de Sante (HAS), France -- national health technology assessment authority · European Union (France) · accessed 2026-07-30 · source 3
- MINIPRESS (prazosin hydrochloride) and TEVA-PRAZOSIN Product MonographsHealth Canada product monographs · Canada · accessed 2026-07-30 · source 4
- MINIPRESS prazosin 1 mg / 2 mg / 5 mg tablets (ARTG 10756, 10757, 10758)Therapeutic Goods Administration (TGA), Australia · Australia · accessed 2026-07-30 · source 5
- APO-PRAZOSIN (1 mg, 2 mg, and 5 mg Tablets) New Zealand Data SheetMedsafe (New Zealand regulator) · New Zealand · accessed 2026-07-30 · source 6
Brand names by country
Single-ingredient prazosin hydrochloride names are shown separately by country. Product availability, strengths, and supply rules can change; combination products are not included here.
Priority countries
United States
- Prazosin hydrochloride (generic; originator Minipress discontinued) (1)FDA approved currently labeled · 1 mg, 2 mg, 5 mg · multiple generic manufacturers
United Kingdom
- Hypovase (2)MHRA licensed emc smpc confirmed · 0.5 mg · Pfizer Limited
Canada
- MINIPRESS
- APO-PRAZO
Australia
New Zealand
- APO-PRAZOSIN (6)Medsafe registered confirmed · 1 mg, 2 mg, 5 mg · Apotex