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Prazosin Side Effects

In Summary

Commonly reported side effects of prazosin include: dizziness. Other side effects include: syncope and drowsiness. See below for a comprehensive list of adverse effects.

For the Consumer

Applies to prazosin: oral capsule, oral tablet

As well as its needed effects, prazosin may cause unwanted side effects that require medical attention.

Major Side Effects

If any of the following side effects occur while taking prazosin, check with your doctor immediately:

More common:
  • Dizziness or lightheadedness, especially when getting up from a lying or sitting position
  • fainting (sudden)
Less common:
  • Loss of bladder control
  • pounding heartbeat
  • swelling of the feet or lower legs
  • Chest pain
  • trouble breathing
Incidence not known:
  • Painful or prolonged erection of the penis

Minor Side Effects

Some prazosin side effects may not need any medical attention. As your body gets used to the medicine these side effects may disappear. Your health care professional may be able to help you prevent or reduce these side effects, but do check with them if any of the following side effects continue, or if you are concerned about them:

More common:
  • Drowsiness
  • headache
  • lack of energy
Less common:
  • Dryness of the mouth
  • nervousness
  • unusual tiredness or weakness
  • Frequent urge to urinate
  • nausea

For Healthcare Professionals

Applies to prazosin: compounding powder, oral capsule


Although all patients are susceptible to prazosin-induced postural hypotension, risk factors include higher doses, coadministration of beta-blockers, and preexisting serum sodium depletion.

Animal studies have revealed that prazosin exerts a negative chronotropic action on the sinus node of the dog. Rare evidence of associated bradycardia in man has been reported. A 50-year-old man with benign prostatic hypertrophy developed presyncope, described as "lightheadedness within 45 minutes after taking prazosin". The patient was referred for pacemaker placement. The patient was convinced that the drug was the offending agent, so he stopped it and became asymptomatic. A follow-up electrocardiogram revealed normal sinus rhythm at 55 beats per minute, and the pacemaker placement was cancelled. The author of this case report has located two other such cases that were reported to the United Kingdom Committee on Safety of Medicines over a 30-year period.[Ref]

Cardiovascular side effects are common during initial therapy. A postural hypotension "first dose effect" has occurred in up to 10% to 70% of patients. For this reason, it is recommended that therapy be started with the patient supine, preferably at bedtime. Syncope has been reported in approximately 1% of patients. Other side effects have included palpitations in 7% to 14% of patients, fluid retention in 5% of patients, and rare case reports of angina and sinus bradycardia. Palpitations may have been due to reflex tachycardia secondary to prazosin-induced decreased peripheral vascular resistance and systemic blood pressure.[Ref]


Ocular side effects including Intraoperative Floppy Iris Syndrome (IFIS) have been observed in some patients undergoing phacoemulsification cataract surgery while being treated with alpha-1 blockers.[Ref]

Most reports were in patients treated with an alpha-1 blocker at the time IFIS occurred, but in some instances the alpha-1 blocker had been stopped prior to surgery. The manufacturer recommends that patients be questioned to determine whether or not they have taken alpha-1 blockers prior to being considered for cataract surgery. If it is determined that the patient has taken an alpha-1 blocker, the patient's ophthalmologist should be prepared for possible modifications to their surgical technique that may be necessary should IFIS be observed during the procedure.[Ref]

Nervous system

Hallucinations, irritability, and confusion have been reported in less than 1% of patients. A rare case of hypothermia has been associated with the use of prazosin.[Ref]

Nervous system side effects have included headache in 8% to 18% (more common in patients with a history of migraine), fatigue in 8%, and blurred vision in less than 5% of patients.[Ref]


Genitourinary side effects have often been due to alpha-adrenergic blockade of the urinary sphincter muscle.[Ref]

Genitourinary side effects have included urinary frequency and incontinence. Impotence has been reported in less than 1% of male patients. Priapism has been reported in a few cases, and may be more likely in patients with renal insufficiency.[Ref]


Rare cases of fecal incontinence, thought to be due to alpha-adrenergic blockade of the anal sphincter due to prazosin, have been reported.[Ref]

Gastrointestinal side effects have included nausea in about 5% of patients. Constipation or diarrhea has occurred in less than 4% of patients. Xerostomia has been less common than with less selective alpha-adrenergic receptor blockers.[Ref]


Hypersensitivity reactions have been rare, and have included urticaria and a serum sickness-like reaction manifest as fever, arthralgias, and cold sweats.[Ref]


Respiratory side effects have been limited to nasal congestion in less than 4% of patients.[Ref]


Musculoskeletal side effects including rare cases of musculoskeletal complaints and acute febrile polyarthritis have been associated with the use of prazosin. The suspected mechanism is immunologic hypersensitivity.[Ref]


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7. Curtis JR, Bateman FJ "Use of prazosin in management of hypertension in patients with chronic renal failure and in renal transplant recipients." Br Med J 4 (1975): 432-4

8. Salim SS, Mtui EP, Makene WJ "An open evaluation of the efficacy and toleration of prazosin in patients with hypertension." East Afr Med J 54 (1977): 429-33

9. Charness ME, Fishman JA, Robertson D "Exacerbation of angina pectoris by prazosin." South Med J 72 (1979): 1213-4

10. "Prazosin (Minipress) for hypertension." Med Lett Drugs Ther 19 (1977): 1-2

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13. Hankey GJ, Gubbay SS "Focal cerebral ischaemia and infarction due to antihypertensive therapy." Med J Aust 146 (1987): 412-4

14. Glass AR, Ballou R "Pheochromocytoma, prazosin, and hypotension." Ann Intern Med 97 (1982): 455

15. Young JB, Leon CA, Pratt CM "Potentially deleterious effects of long-term vasodilator therapy in patients with heart failure." Chest 91 (1987): 737-44

16. Kobrin I, Stessman J, Yagil Y, Ben-Ishay D "Prazosin-induced bradycardia in acute treatment of hypertension." Arch Intern Med 143 (1983): 2019,2023

17. Guilleminault C, Mignot E, Aldrich M, Quera-Salva MA, Tiberge M, Partinen M "Prazosin contraindicated in patients with narcolepsy." Lancet 2 (1988): 511

18. Chin DK, Ho AK, Tse CY "Neuropsychiatric complications related to use of prazosin in patients with renal failure." Br Med J 293 (1986): 1347

19. Patterson JF "Auditory hallucinations induced by prazosin." J Clin Psychopharmacol 8 (1988): 228

20. Aldrich MS, Rogers AE "Exacerbation of human cataplexy by prazosin." Sleep 12 (1989): 254-6

21. Singleton W, Dix RK, Monsen L, et al "Efficacy and safety of minipress XL, a new once-a-day formulation of prazosin." Am J Med 87 (1989): s45-52

22. De Leeuw PW, Birkenhager WH "Hypothermia: a possible side effect of prazosin." Br Med J 281 (1980): 1181

23. Thien T, Delaere KP, Debruyne FM, Koene RA "Urinary incontinence caused by prazosin." Br Med J 1 (1978): 622-3

24. Ylitalo P, Pasternack A "Priapism: side-effect of prazosin in patients with renal failure." Acta Med Scand 213 (1983): 319-20

25. Smith PJ, Talbert RL "Sexual dysfunction with antihypertensive and antipsychotic agents." Clin Pharm 5 (1986): 373-84

26. Dwyer PL, Teele JS "Prazosin: a neglected cause of genuine stress incontinence." Obstet Gynecol 79 (1992): 117-21

27. Holmes SA, Christmas TJ, Wood JJ, Kirby RS "Faecal incontinence resulting from a1-adrenoceptor blockade." Lancet 336 (1990): 685-6

28. Ruzicka T, Ring J "Hypersensitivity to prazosin." Lancet Feb (1983): 473-4

29. Cairns SA, Jordan SC "Prazosin treatment complicated by acute febrile polyarthritis." Br Med J 2 (1976): 1424-5

30. Bell A "Polyarthralgia associated with prazosin use." Ann Pharmacother 29 (1995): 937

It is possible that some side effects of prazosin may not have been reported. These can be reported to the FDA here. Always consult a healthcare professional for medical advice.

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