Minipress Side Effects

Generic name: prazosin

Note: This document contains side effect information about prazosin. Some of the dosage forms listed on this page may not apply to the brand name Minipress.

Some side effects of Minipress may not be reported. Always consult your doctor or healthcare specialist for medical advice. You may also report side effects to the FDA.

For the Consumer

Applies to prazosin: oral capsule

Get emergency medical help if you have any of these signs of an allergic reaction while taking prazosin (the active ingredient contained in Minipress) hives; difficulty breathing; swelling of your face, lips, tongue, or throat.

Call your doctor at once if you have a serious side effect such as:

• fast or pounding heartbeats or fluttering in your chest;

• feeling like you might pass out;

• trouble breathing;

• swelling in your hands, ankles, or feet; or

• penis erection that is painful or lasts 4 hours or longer.

Less serious side effects of prazosin may include:

• mild dizziness;

• weakness, tired feeling, drowsiness;

• headache; or

• nausea.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects.

For Healthcare Professionals

Applies to prazosin: compounding powder, oral capsule

Cardiovascular

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 (the active ingredient contained in Minipress) 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.

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.

Ocular

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.

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.

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 (the active ingredient contained in Minipress)

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.

Genitourinary

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

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.

Gastrointestinal

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.

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

Hypersensitivity

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

Respiratory

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

Musculoskeletal

Musculoskeletal side effects including rare cases of musculoskeletal complaints and acute febrile polyarthritis have been associated with the use of prazosin (the active ingredient contained in Minipress) The suspected mechanism is immunologic hypersensitivity.

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