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, hypertension.
Endocrine: gynecomastia.
Heart Rate/Rhythm: bradycardia.
Psychiatric: insomnia.
Skin/Appendages: urticaria.
Vascular (Extracardiac):vasculitis.
Vision: eye pain.

MINIPRESS (prazosin hydrochloride) 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 (see WARNINGS however), 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.

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.

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