Polythiazide / prazosin Side Effects

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

For the Consumer

Applies to polythiazide / prazosin: oral capsule

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

If any of the following side effects occur while taking polythiazide / prazosin, check with your doctor or nurse as soon as possible:

Signs and symptoms of too much potassium loss
  • Dryness of mouth (severe)
  • increased thirst
  • irregular heartbeat (continuing)
  • mood or mental changes
  • muscle cramps or pain
  • nausea or vomiting
  • unusual tiredness or weakness
  • weak pulse
Signs and symptoms of too much sodium loss
  • Confusion
  • convulsions
  • decreased mental activity
  • irritability
  • muscle cramps
  • unusual tiredness or weakness
More common
  • Dizziness or lightheadedness, especially when getting up from a lying or sitting position
  • sudden fainting
Less common
  • Inability to control urination
  • irregular heartbeat
  • pounding heartbeat
  • swelling of feet or lower legs
  • weight gain
  • Black, tarry stools
  • blood in urine or stools
  • chest pain
  • cough or hoarseness
  • fever or chills
  • joint pain
  • lower back or side pain
  • painful or difficult urination
  • painful, inappropriate erection of penis, continuing
  • pinpoint red spots on skin
  • shortness of breath
  • skin rash or hives
  • stomach pain (severe) with nausea and vomiting
  • unusual bleeding or bruising
  • yellow eyes or skin

Some polythiazide / 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:

Less common
  • Decreased sexual ability
  • diarrhea
  • drowsiness
  • headache
  • increased sensitivity of skin to sunlight
  • lack of energy
  • loss of appetite
  • nervousness
  • stomach upset or pain
  • Frequent urge to urinate
  • nausea

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.

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.

Nervous system

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.

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.


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).

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.


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).


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.

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


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.


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.


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.

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.


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.

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.


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.

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.


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


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

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