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Minizide Side Effects

Generic name: polythiazide / prazosin

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

Applies to polythiazide / prazosin: oral capsule.

Serious side effects of Minizide

Along with its needed effects, polythiazide / prazosin may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

Check with your doctor as soon as possible if any of the following side effects occur while taking polythiazide / prazosin:

Signs and symptoms of too much potassium loss

Signs and symptoms of too much sodium loss

More common

Less common


Other side effects of Minizide

Some side effects of polythiazide / prazosin may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects.

Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:

Less common


For Healthcare Professionals

Applies to polythiazide / prazosin: oral capsule.


Cardiovascular side effects are common during initial therapy. A postural hypotension "first dose effect" occurs in up to 10% to 70% of patients who are taking prazosin. For this reason, it is recommended that therapy be started with the patient supine, preferably at bedtime. Dizziness or syncope is reported in 10% and 1% of patients, respectively. Other side effects include palpitations in 7% to 14% of patients, fluid retention in 5% of patients, and rare cases of angina or sinus bradycardia. Cardiac arrhythmias, including ventricular ectopy and complete AV heart block, have been associated with thiazide-induced hypokalemia.[Ref]

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

Animal studies reveal that prazosin exerts a negative chronotropic action on the sinus node of the dog. There is evidence that this may be important in humans. 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, but, because he was convinced that the drug was causing his problem, 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]

Nervous system

Nervous system side effects are most likely due to prazosin, and include 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]

Hallucinations, irritability, and confusion have been reported in less than 1% of patients who were taking prazosin. A rare cases of hypothermia has been associated with prazosin.[Ref]


The metabolic side effects of polythiazide, as with other thiazide diuretics, include hypokalemia, hyponatremia, hypochloremia, hypercalcemia, hypercholesterolemia, and hyperuricemia. Significant reductions in the serum potassium concentration (decreases of 0.5 mEq/L or more) have been observed in up to 50% of patients who are taking moderate doses of thiazide diuretics. This reduction in serum potassium can predispose some patients to develop cardiac arrhythmias.[Ref]

Some of the metabolic changes associated with thiazides may be significant in patients with underlying cardiac arrhythmias (hypokalemia), coronary artery disease (hypercholesterolemia), gout (hyperuricemia), or liver disease (hyponatremia or hypokalemia).[Ref]


Hypersensitivity reactions to thiazide diuretics are reported in less than 1% of patients. While most allergic reactions present as rash with nausea and vomiting, rare cases of acute pulmonary edema, interstitial cystitis, interstitial nephritis, and anaphylaxis have been associated with the use of some thiazides. Hypersensitivity reactions to prazosin are rare, and include urticaria and a single report of a serum sickness-like reaction (fever, arthralgias, cold sweats).[Ref]


Urinary incontinence associated with prazosin is due to alpha-adrenergic blockade of the urinary sphincter.[Ref]

Genitourinary symptoms associated with prazosin include urinary frequency or incontinence. Impotence occurs in less than 1% of male patients. Priapism has rarely been associated with prazosin, and may be more likely in patients with renal insufficiency.[Ref]


Dermatologic reactions to thiazides include erythema annular centrifugum, acute eczematous dermatitis, and morbilliform or leukocytoclastic vasculitis. Thiazides may induce phototoxic dermatitis. In addition, a rare, distinct entity with clinical and laboratory features indistinguishable from those of subacute cutaneous lupus erythematosus has been associated with a related drug, hydrochlorothiazide.[Ref]


Renal side effects including new or worsened renal insufficiency may occur due to polythiazide-induced intravascular volume depletion. Rare cases of interstitial nephritis have been associated with the use of some thiazides.[Ref]


Endocrinologic problems associated with thiazide diuretics include glucose intolerance and a potentially deleterious effect on the lipid profile. This may be important in some patients with or who are at risk for diabetes or coronary artery disease.[Ref]

A prospective study of 34 patients who received oral thiazide diuretics for 14 years without interruption revealed an increased mean fasting blood glucose level after treatment. Withdrawal of thiazide therapy for 7 months in 10 of the patients resulted in mean reductions of 10% in fasting blood glucose and 25% in the 2-hour glucose tolerance test values. A control group was not reported.[Ref]


Hematologic side effects are rare. Cases of immune-complex hemolytic anemia, aplastic anemia, and thrombocytopenia have been associated either with polythiazide or related thiazide agents.[Ref]

A 2.5-year-old male with anasarca developed cyanosis and oral ulcerations association with laboratory evidence of pancytopenia 15 days after beginning polythiazide (dose not available). A bone marrow aspiration revealed general hypoplasia with absence of megakaryocytes and blast cells. The child died from persistent hemorrhaging and infections despite prednisolone and antimicrobial therapy.[Ref]


Gastrointestinal side effects include nausea in approximately 5% of patients. Constipation and diarrhea occur in less than 4% of patients. Xerostomia is less commonly seen with prazosin than with less selective alpha-adrenergic receptor blockers. There have been rare cases of pancreatitis and acute cholecystitis associated with thiazide diuretics.[Ref]

Thiazide diuretics may increase serum cholesterol and triglycerides, resulting in an increased risk of cholesterol gallstone formation. Reports of bowel strictures associated with thiazide ingestion have been reported in the 1960's, although these patients were taking a combination hydrochlorothiazide-potassium product.

A single case of fecal incontinence, thought to be due to alpha-adrenergic blockade of the anal sphincter (due to prazosin) has been reported.[Ref]


Respiratory system side effects associated with prazosin have included nasal congestion, seen in less than 4% of patients.[Ref]


Psychiatric side effects including depression have been associated with the use of thiazide diuretics. These data are uncontrolled observations and have not been substantiated.[Ref]


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Further information

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

Some side effects may not be reported. You may report them to the FDA.