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Prazosin: 7 things you should know

Medically reviewed by Carmen Fookes, BPharm. Last updated on June 29, 2021.

1. How it works

  • Prazosin may be used to reduce high blood pressure.
  • The exact way prazosin works is unknown; however, experts believe it relaxes the smooth muscle lining the blood vessels, allowing the blood vessels to widen (dilate). Prazosin may also block nerve receptors responsible for contracting (narrowing) blood vessels.
  • Prazosin belongs to the group of medicines known as alpha-1 adrenergic blockers (may be shortened to alpha-blockers).

2. Upsides

  • May be used to treat high blood pressure (hypertension).
  • Lowers blood pressure without causing a compensatory reflex tachycardia (this is a counteracting fast heart rate - often a physiological response to blood pressure lowering), or a change in the contraction of the heart or the flow of blood through the kidneys.
  • Trials have shown that lowering blood pressure reduces the risk of a stroke, myocardial infarction (heart attack), and other cardiovascular events.
  • The blood pressure lowering effects of prazosin remain consistent even with long-term use.
  • Blood pressure-lowering effects occur while standing or lying down.
  • Does not adversely affect blood lipid (cholesterol) levels.
  • May be better tolerated by people with diabetes, respiratory diseases, or gout than some other antihypertensive agents.
  • Can be used alone or in combination with other antihypertensive agents such as diuretics and beta-blockers.
  • Side effects are usually mild and short-lived.
  • Available as 1mg, 2mg, and 5mg capsules.
  • Generic prazosin is available.

3. Downsides

If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include:

  • Dizziness or drowsiness that may affect your ability to drive or operate machinery, particularly within the first 24 hours of dosing, when the dose is increased, when going from a lying down to a standing position, during hot weather, after exercise, or after drinking alcohol. Do not drive or operate machinery until you know how prazosin affects you. Getting up slowly may help lessen the problem.
  • A headache, lack of energy, weakness, nausea, and palpitations may also occur.
  • May rarely cause syncope (fainting, or a temporary loss of consciousness), especially when going from a lying or sitting position to a standing position. These episodes may occur within 30 to 90 minutes of taking prazosin and the risk of an episode is higher during a dosage increase or when prazosin is used in combination with another antihypertensive drug. Incidence is approximately 1% in people prescribed dosages of 2mg or greater. Incidence may be reduced by limiting the initial dose to 1mg and titrating slowly.
  • Sexual dysfunction is uncommon. Rarely, may cause prolonged erections lasting more than four hours. Seek immediate medical help.
  • May not be suitable for some people including those with low blood pressure, about to undergo eye surgery, or who are pregnant or breastfeeding.
  • Safety and effectiveness in children has not been established.
  • Intraoperative floppy iris syndrome has been observed during cataract surgery in some people treated with alpha-1-blockers. Modifications may need to be made to the surgical technique because there does not appear to be a benefit of stopping alpha-1-blocker therapy before surgery.
  • Prazosin may interact with some drugs, including those that also lower blood pressure or cause dizziness.
  • The safety of prazosin in pregnant women has not been established. Only use if the potential benefit justifies the risk. Use caution if prazosin is used in a breastfeeding woman.

Note: In general, seniors or children, people with certain medical conditions (such as liver or kidney problems, heart disease, diabetes, seizures) or people who take other medications are more at risk of developing a wider range of side effects. View complete list of side effects

4. Bottom Line

Prazosin effectively treats high blood pressure but commonly causes dizziness or lightheadedness on initiation or with a dosage increase. However, a tolerance to this effect develops on continued use.

5. Tips

  • May be taken with or without food.
  • Take exactly as directed by your doctor. Do not take more or less prazosin than recommended.
  • If you feel yourself beginning to faint or feeling dizzy or lightheaded while on prazosin, lie down until the feeling passes. This effect is usually short-lived and likely only to occur on therapy initiation or during dose titration. Be careful with the amount of alcohol you drink when on prazosin as alcohol may increase the risk of becoming dizzy or lightheaded. Do not drive or operate machinery if you feel impaired as a result of taking prazosin.
  • Initial dosages may be better taken at bedtime.
  • Good blood pressure control requires lifestyle changes to also be made, such as stopping smoking, reducing dietary sodium intake, and increasing levels of physical activity. In addition, blood sugars and lipids should be kept within normal limits and antithrombotic (blood-thinning) therapy should be considered for certain people. Many patients require more than one drug to achieve blood pressure targets.
  • Talk to your doctor if prazosin makes you extremely dizzy, if you are falling over easily, if you suffer a sustained erection of more than a few hours duration, or develop unusual heart palpitations.
  • Do not take any other medication, including that bought over the counter, without first checking with your doctor or pharmacist that it is compatible with prazosin.

6. Response and effectiveness

Blood levels of prazosin reach a peak within about three hours of an oral dose. The blood pressure-lowering effect occurs within two hours of a dose and reaches a peak at two to four hours. Prazosin needs to be taken two to three times daily.

7. Interactions

Medicines that interact with prazosin may either decrease its effect, affect how long it works for, increase side effects, or have less of an effect when taken with prazosin. An interaction between two medications does not always mean that you must stop taking one of the medications; however, sometimes it does. Speak to your doctor about how drug interactions should be managed.

Common medications that may interact with prazosin include:

  • antidepressants, such as amitriptyline, clomipramine, desipramine, doxepin, escitalopram, imipramine, or nortriptyline
  • antipsychotics, such as aripiprazole, haloperidol, olanzapine, thioridazine, or ziprasidone
  • aspirin and NSAIDs such as ibuprofen, diclofenac, and naproxen
  • barbiturates, such as phenobarbital
  • benzodiazepines, such as alprazolam, diazepam, oxazepam, and temazepam
  • bupropion
  • corticosteroids, such as betamethasone, cortisone, or prednisone
  • diuretics, such as bendroflumethiazide, furosemide
  • erectile dysfunction medications, such as sildenafil or tadalafil
  • heart medications, such as atenolol, or sotalol
  • house dust mite allergen extract
  • licorice
  • lithium
  • mixed grass pollens allergen extract
  • monoamine oxidase inhibitors, such as isocarboxazid, selegiline, or tranylcypromine
  • muscle relaxants, such as baclofen
  • opioids, such as oxycodone, methadone, morphine, or codeine
  • Parkinson's disease medications, such as selegiline
  • peanut allergen extract
  • ragweed pollen allergen extract
  • tizanidine.

May cause false-positive results in screening tests for pheochromocytoma.

Note that this list is not all-inclusive and includes only common medications that may interact with prazosin. You should refer to the prescribing information for prazosin for a complete list of interactions.


Prazosin Revised 03/2021.

Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use prazosin only for the indication prescribed.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Copyright 1996-2022 Revision date: June 28, 2021.