Side Effects of Minipress (prazosin)

Minipress (prazosin) is an alpha-1 adrenergic receptor blocker used to treat high blood pressure (hypertension).

By blocking alpha-1 receptors on muscle cells that surround blood vessels, Minipress causes vasodilation (widening) of the blood vessels, and consequently decreases the resistance of blood flow. The overall benefit of its use is a decrease in blood pressure.

Minipress seems to have a bigger impact on reducing the diastolic blood pressure than systolic blood pressure. Diastolic blood pressure is indicated by the second set of numbers in the blood pressure reading and corresponds to the minimum pressure in the arteries when the heart muscles are relaxed and the chambers of the heart are filling with blood. 

Common side effects of Minipress include

• dizziness,
• headache,
• drowsiness,
• lack of energy,
• weakness,
• palpitations, and
• nausea.

Less common side effects of Minipress include

• vomiting,
• diarrhea,
• constipation,
• fluid retention (edema),
• dizziness on standing,
• shortness of breath,
• fainting,
• motion sickness,
• depression,
• nervousness,
• rash,
• urinary frequency,
• blurred vision,
• eye redness,
• nosebleeds,
• dry mouth, and
• nasal congestion.

Rare side effects of Minipress include

• stomach pain,
• liver problems,
• pancreatitis,
• fast heartbeat,
• numbness/tingling/burning/prickling,
• hallucinations,
• itching,
• hair loss,
• urinary incontinence,
• sexual dysfunction,
• prolonged erection,
• ringing in the ears,
• sweating,
• fever, and
• joint pain.

Drug interactions of Minipress include other blood pressure lowering medicines or water pills, because it may cause an additive blood pressure lowering effect. Use of phosphodiesterase-5 (PDE-5) inhibitors with Minipress can also cause an additive decrease in blood pressure. 

There are no adequate and well-controlled studies of Minipress use in pregnancy. Minipress should be used during pregnancy only if the potential benefit justifies the potential risk to the mother and the fetus.

Small amounts of Minipress were found in human milk after oral administration. Due to the lack of conclusive safety data, Minipress should be used cautiously in breastfeeding mothers.

The most common side effects associated with prazosin treatment include:

• dizziness,
• headache,
• drowsiness,
• lack of energy,
• weakness,
• palpitations, and
• nausea.

Patients can lower their chance of feeling dizzy or passing out by:

• rising slowing from a sitting or lying position,
• climbing stairs slowly,
• avoiding alcohol, and
• drinking lots of water especially in hot weather or while being active.

Additionally, patients should have their blood pressure checked regularly.

Less common side effects include:

• vomiting,
• diarrhea,
• constipation,
• water retention (edema),
• orthostatic hypotension,
• shortness of breath,
• fainting (syncope),
• motion sickness,
• depression,
• nervousness,
• rash,
• urinary frequency,
• blurred vision,
• reddened sclera,
• nosebleeds,
• dry mouth, and nasal
• congestion.

Rare side effects include:

• stomach pain,
• liver problems,
• pancreatitis,
• tachycardia (fast heartbeat),
• paresthesia (numbness, tingling, burning, prickling),
• hallucinations,
• itching,
• hair loss,
• urinary incontinence (loss of bladder control),
• sexual dysfunction,
• prolonged erection,
• ringing in the ears,
• sweating,
• fever, and
• joint pain.

Other side effects reported in post-marketing trials include:

• allergic reaction,
• weakness,
• pain,
• chest pain,
• low blood pressure,
• gynecomastia (enlargement of mens' breasts, bradycardia [slow heartbeat]),
• difficulty sleeping,
• hives,
• vasculitis, and
• eye pain. 

Clinical trials were conducted on more than 900 patients. During these trials and subsequent marketing experience, the most frequent reactions associated with Minipress therapy are:

• dizziness 10.3%,
• headache 7.8%,
• drowsiness 7.6%,
• lack of energy 6.9%,
• weakness 6.5%,
• palpitations 5.3%, and
• nausea 4.9%.

In most instances, side effects have disappeared with continued therapy or have been tolerated with no decrease in dose of drug.

Less frequent adverse reactions which are reported to occur in 1-4% of patients are:

Gastrointestinal: vomiting, diarrhea, constipation.

Cardiovascular: edema, orthostatic hypotension, dyspnea, syncope.

Central Nervous System: vertigo, depression, nervousness.

Dermatologic: rash.

Genitourinary: urinary frequency.

EENT: blurred vision, reddened sclera, epistaxis, dry mouth, nasal congestion.

In addition, fewer than 1% of patients have reported the following (in some instances, exact causal relationships have not been established):

Gastrointestinal: abdominal discomfort and/or pain, liver function abnormalities, pancreatitis.

Cardiovascular: tachycardia.

Central Nervous System: paresthesia, hallucinations.

Dermatologic: pruritus, alopecia, lichen planus.

Genitourinary: incontinence, impotence, priapism.

EENT: tinnitus.

Other: diaphoresis, fever, positive ANA titer, arthralgia.

Single reports of pigmentary mottling and serous retinopathy, and a few reports of cataract development or disappearance have been reported. In these instances, the exact causal relationship has not been established because the baseline observations were frequently inadequate.

In more specific slit-lamp and funduscopic studies, which included adequate baseline examinations, no drug-related abnormal ophthalmological findings have been reported.

Literature reports exist associating Minipress therapy with a worsening of pre-existing narcolepsy. A causal relationship is uncertain in these cases.

In post-marketing experience, the following adverse events have been reported:

Autonomic Nervous System: flushing.

Body As A Whole: allergic reaction, asthenia, malaise, pain.

Cardiovascular, General: angina pectoris, hypotension.

Endocrine: gynecomastia.

Heart Rate/Rhythm: bradycardia.

Psychiatric: insomnia.

Skin/Appendages: urticaria.

Vascular (Extracardiac): vasculitis.

Vision: eye pain.

Special Senses: During cataract surgery, a variant of small pupil syndrome known as Intraoperative Floppy Iris Syndrome (IFIS) has been reported in association with alpha-1 blocker therapy.

Minipress has been administered without any adverse drug interaction in limited clinical experience to date with the following:

• (1) cardiac glycosides-digitalis and digoxin;
• (2) hypoglycemics-insulin, chlorpropamide, phenformin, tolazamide, and tolbutamide;
• (3) tranquilizers and sedatives- chlordiazepoxide, diazepam, and phenobarbital;
• (4) antigout-allopurinol, colchicine, and probenecid;
• (5) antiarrhythmics-procainamide, propranolol, and quinidine; and (6) analgesics, antipyretics and anti-inflammatories-propoxyphene, aspirin, indomethacin, and phenylbutazone.

Addition of a diuretic or other antihypertensive agent to Minipress has been shown to cause an additive hypotensive effect. This effect can be minimized by reducing the Minipress dose to 1 to 2 mg three times a day, by introducing additional antihypertensive drugs cautiously, and then by retitrating Minipress based on clinical response.

Concomitant administration of Minipress with a phosphodiesterase-5 (PDE-5) inhibitor can result in additive blood pressure lowering effects and symptomatic hypotension.

Drug/Laboratory Test Interactions

In a study on five patients given from 12 to 24 mg of prazosin per day for 10 to 14 days, there was an average increase of 42% in the urinary metabolite of norepinephrine and an average increase in urinary VMA of 17%.

Therefore, false positive results may occur in screening tests for pheochromocytoma in patients who are being treated with prazosin. If an elevated VMA is found, prazosin should be discontinued and the patient retested after a month.

Laboratory Tests

In clinical studies in which lipid profiles were followed, there were generally no adverse changes noted between pre- and post-treatment lipid levels.