Prazosin Blood Pressure Uses Warnings Side Effects Dosage

Prazosin Blood Pressure Uses Warnings Side Effects Dosage


Prazosin is an antihypertensive medication used to treat high blood pressure (hypertension), either alone or in combination with other antihypertensive drugs. Prazosin reduces blood pressure by relaxing the smooth muscles around blood vessels, improving blood flow. It is also used off-label for enlarged prostate (benign prostate hyperplasia), posttraumatic stress disorder (PTSD), and Raynaud’s phenomenon.

Prazosin belongs to the alpha blocker class of medications and is derived from quinazoline. It binds to alpha-1 receptors on smooth muscles, preventing their stimulation by epinephrine and norepinephrine. This relaxes the smooth muscles in blood vessels, prostate, urethra, iris dilator muscle, and the brain.

Prazosin improves urinary flow in patients with benign prostate hyperplasia by relaxing the smooth muscles in the bladder neck and prostate. It also improves peripheral blood flow in patients with Raynaud’s phenomenon. In addition, it crosses the blood-brain barrier and reduces PTSD-related hyperarousal states by inhibiting catecholamine activity in the brain.


  • Do not administer prazosin to patients with known sensitivity to quinazoline, prazosin, or any of its components
  • Discontinue prazosin if the patient has onset or worsening of chest pain (angina)
  • Exercise caution in patients with heart failure, as prazosin may exacerbate the condition
  • Prazosin can cause temporary loss of consciousness (syncope) due to vasodilation and drop in blood pressure, especially within the first 30-90 minutes after the first dose; keep the patient lying flat and provide supportive care
  • Exercise caution when administering prazosin with other vasodilators or in patients with narcolepsy or sleep disorder
  • May cause central nervous system (CNS) depression; caution patients appropriately
  • May cause prolonged and painful erections (priapism); advise patients appropriately
  • Rule out prostate cancer before initiating prazosin therapy
  • Use appropriate technique during cataract surgery in patients treated with alpha-1 blockers to prevent intraoperative floppy iris syndrome
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What are the side effects of prazosin?

Common side effects of prazosin include:

Less common side effects of prazosin include:

  • Diarrhea
  • Constipation
  • Vomiting
  • Dry mouth (xerostomia)
  • Swelling (edema)
  • Shortness of breath (dyspnea)
  • Orthostatic hypotension (drop in blood pressure when standing up)
  • Fainting (syncope)
  • Vertigo
  • Depression
  • Nervousness
  • Rash
  • Urinary frequency
  • Nasal congestion
  • Nasal bleeding (epistaxis)
  • Blurred vision
  • Red eye

Rare side effects of prazosin include:

  • Abdominal discomfort and/or pain
  • Liver function abnormalities
  • Pancreas inflammation (pancreatitis)
  • Rapid heart rate (tachycardia)
  • Chest pain (angina pectoris)
  • Low blood pressure (hypotension)
  • Slow heart rate (bradycardia)
  • Hallucinations
  • Sleeplessness (insomnia)
  • Abnormal skin sensations (paresthesia)
  • Ringing in the ears (tinnitus)
  • Eye pain
  • Allergic reaction
  • Hives (urticaria)
  • Itching (pruritus)
  • Lichen planus
  • Autoimmune reaction (positive ANA titer)
  • Hair loss (alopecia)
  • Incontinence
  • Impotence
  • Prolonged and painful erection (priapism)
  • Excessive sweating (diaphoresis)
  • Flushing
  • Fever
  • Joint pain (arthralgia)
  • Feeling unwell (malaise)
  • Generalized pain
  • Blood vessel inflammation (vasculitis)
  • Breast tissue growth in males (gynecomastia)
  • Intraoperative floppy iris syndrome during cataract surgery

This is not a complete list of all side effects or adverse reactions that may occur. Please consult your doctor or report any serious side effects or adverse reactions to the FDA at 1-800-FDA-1088.

What are the dosages of prazosin?



  • Initial: 1 mg orally every 8-12 hours
  • Maintenance: 6-15 mg/day divided 2 or 3 times daily; alternatively, 1-5 mg orally twice daily; may increase the dose to 20 mg/day in divided doses; some patients may benefit from up to 40 mg/day in divided doses

PTSD-Related Nightmares and Sleep Disruption (off label)

  • Initial: 1 mg orally at bedtime
  • Maintenance: 1 mg orally at bedtime initially; may increase the dose to 2 mg at bedtime; adjust dose based on response and tolerability in 1-2 mg increments every 7 days; not to exceed 15 mg/day
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Benign Prostate Hypertrophy (off label)

  • Initial: 0.5 mg orally every 12 hours
  • Maintenance: 2 mg orally every 12 hours

Raynaud Phenomenon (off label)

  • 0.5-1 mg orally each day (at bedtime) or 0.5 mg orally twice daily; adjust dose based on response and tolerability up to 12 mg/day divided twice daily/three times daily


  • Give first dose and subsequent increases at bedtime to avoid syncope
  • Okay with food


  • Avoid use for hypertension due to high risk of orthostatic hypotension
  • May cause significant orthostatic hypotension and syncope in older adults; consider lower initial dose and titrate to response
  • Adverse effects such as dry mouth and urinary complications can be bothersome in the elderly


Hypertension (off label)

  • Initial: 0.05-0.1 mg/kg/day orally divided every 8 hours
  • Titrate to 0.5 mg/kg/day; not to exceed 20 mg/day



  • Prazosin overdose may cause drowsiness, depressed reflexes, and low blood pressure (hypotension)
  • Overdose is treated with symptomatic and supportive care, including increasing the blood volume and normalizing blood pressure with vasopressor medications
  • In case of overdose, seek medical help immediately or contact Poison Control

What drugs interact with prazosin?

Inform your doctor of all medications you are currently taking, as there may be potential drug interactions. Do not begin, discontinue, or change the dosage of any medication without your doctor’s recommendation.

  • Prazosin has no known severe interactions with other drugs.
  • Serious interactions of prazosin include: bremelanotide, lofexidine, sildenafil, tamsulosin, vardenafil, and yohimbe.
  • Other medications with interactions include brimonidine, butcher’s broom, ethanol, phenylephrine, phenylephrine oral, tizanidine, and treprostinil.

Please note that the interactions listed above are not all of the possible interactions or adverse effects. For more information, use the RxList Drug Interaction Checker. Always inform your doctor, pharmacist, or healthcare provider about all prescription and over-the-counter medications you use.

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What else should I know about prazosin?

  • Take prazosin exactly as prescribed
  • Keep out of reach of children
  • You may experience dizziness or drowsiness, especially after the first dose or after a dose increase. Avoid driving or tasks that require alertness until the effects of the drug can be determined
  • Getting up slowly may help reduce lightheadedness when standing up
  • Dizziness and fainting may occur with alcohol, dehydration, exercise, or standing for long periods. Exercise caution.
  • Seek medical help in case of prolonged and painful erections.

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Pregnancy and breastfeeding

  • Limited studies of prazosin use during pregnancy have not revealed any fetal or neonatal harm. However, there are no adequate and well-controlled studies in pregnant women.
  • Use during pregnancy if the potential benefits to the mother outweigh the potential risk to the mother and fetus.
  • Prazosin is excreted in breast milk in small amounts. Use with caution in nursing mothers.


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Prazosin is an antihypertensive medication used to treat high blood pressure (hypertension), either alone or in combination with other antihypertensive drugs. It is also used off-label for enlarged prostate, posttraumatic stress disorder (PTSD), and Raynaud’s phenomenon. Common side effects include dizziness, headache, drowsiness, fatigue, palpitations, and nausea. Consult your doctor before taking prazosin if pregnant or breastfeeding.


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