prazosin

Prazosin is an antihypertensive medication used in the treatment of high blood pressure (hypertension), either alone or in combination with other antihypertensive drugs such as diuretics or beta blockers. Prazosin reduces blood pressure by relaxing the smooth muscles around blood vessels, dilating them and improving blood flow. Prazosin is also used off-label in the treatment of enlarged prostate (benign prostate hyperplasia), posttraumatic stress disorder (PTSD), and Raynaud’s phenomenon.

Prazosin is derived from a chemical known as quinazoline and belongs to the alpha blocker class of medications. Prazosin binds to alpha-1 receptors, protein molecules on smooth muscles, and prevents their stimulation by the hormones (catecholamines) epinephrine and norepinephrine, which makes the smooth muscles contract. Alpha-1 receptors are present in the smooth muscles of blood vessel walls, 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. Vasodilation improves peripheral blood flow in patients with Raynaud’s phenomenon, a condition that causes poor circulation in the extremities. Prazosin crosses the blood-brain barrier and inhibits hyperarousal states by catecholamine activity in the brain, reducing PTSD-related insomnia and nightmares.

• 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)
• Prazosin may exacerbate heart failure; use with caution
• Prazosin can cause temporary loss of consciousness (syncope) due to vasodilation and drop in blood pressure, especially in the first 30-90 minutes after the first dose; keep the patient lying flat and treat with supportive care as necessary
• Can cause syncope and orthostatic hypotension when first administered after an interruption of therapy or when administered with other vasodilators such as beta blockers or phosphodiesterase-5 (PDE-5) inhibitors; exercise caution
• May cause central nervous system (CNS) depression; caution patients appropriately
• May aggravate symptoms in patients with narcolepsy sleep disorder
• May cause prolonged and painful erections (priapism); advise patients appropriately
• Rule out prostate cancer before initiating prazosin therapy
• Intraoperative floppy iris syndrome has been reported during cataract surgery in some patients treated with alpha-1 blockers; use appropriate technique

Common side effects of prazosin include:

• Dizziness
• Headache
• Drowsiness
• Fatigue
• Weakness (asthenia)
• Palpitations
• Nausea

Less common side effects of prazosin include:

• Diarrhea
• Constipation
• Vomiting
• Dry mouth (xerostomia)
• Swelling (edema)
• Shortness of breath (dyspnea)
• Drop in blood pressure when standing from sitting or lying down (orthostatic hypotension)
• 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, because of relaxation of iris dilator muscle

This is not a complete list of all side effects or adverse reactions that may occur from the use of this drug.

Call your doctor for medical advice about serious side effects or adverse reactions. You may also report side effects or health problems to the FDA at 1-800-FDA-1088.

Capsule

• 1 mg
• 2 mg
• 5 mg

Adult:

Hypertension

• 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

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

Administration

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

Geriatric:

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

Pediatric:

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

Administration:

• Okay with food

• 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 normalization of blood pressure with vasopressor medications.
• In case of overdose, seek medical help immediately or contact Poison Control.

Inform your doctor of all medications you are currently taking, who can advise you on any possible drug interactions. Never begin taking, suddenly 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
  • yohimbe
• Prazosin has moderate interactions with at least 99 different drugs.
• Mild interactions of prazosin include:
  • brimonidine
  • butcher’s broom
  • ethanol
  • phenylephrine
  • phenylephrine oral
  • tizanidine
  • treprostinil

The drug interactions listed above are not all of the possible interactions or adverse effects. For more information on drug interactions, visit the RxList Drug Interaction Checker.

It is important to always tell your doctor, pharmacist, or health care provider of all prescription and over-the-counter medications you use, as well as the dosage for each, and keep a list of the information. Check with your doctor or health care provider if you have any questions about the medication.

• Take prazosin exactly as prescribed
• Keep safely out of reach of children
• You may have dizziness or drowsiness especially after the first dose or after a dose increase; avoid driving and tasks that require physical and mental alertness for the first 24 hours until the effects of the drug can be determined
• Getting up slowly may help reduce lightheadedness that you may feel when standing up from sitting or lying down
• Drinking alcohol, dehydration, exercise or standing for long periods can also cause dizziness and fainting; exercise caution
• Seek medical help in case of prolonged and painful erections

• Limited studies of prazosin use to treat hypertension 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 potential benefits to the mother outweighs potential risk to the mother and fetus.
• Prazosin is excreted in breast milk in small amounts; use with caution in nursing mothers.