Prolonged erections and priapism have been reported with alpha-1 blockers including prazosin in post marketing experience. In the event of an erection that persists longer than 4 hours, seek immediate medical assistance. If priapism is not treated immediately, penile tissue damage and permanent loss of potency could result.
Intraoperative Floppy Iris Syndrome (IFIS) has been observed during cataract surgery in some patients treated with alpha-1 blockers. This variant of small pupil syndrome is characterized by the combination of a flaccid iris that billows in response to intraoperative irrigation currents, progressive intraoperative miosis despite preoperative dilation with standard mydriatic drugs, and potential prolapse of the iris toward the phacoemulsification incisions. The patient’s ophthalmologist should be prepared for possible modifications to the surgical technique, such as the utilization of iris hooks, iris dilator rings, or viscoelastic substances. There does not appear to be a benefit of stopping alpha-1 blocker therapy prior to cataract surgery.
Dizziness or drowsiness may occur after the first dose of this medicine. Avoid driving or performing hazardous tasks for the first 24 hours after taking this medicine or when the dose is increased. Dizziness, lightheadedness, or fainting may occur, especially when rising from a lying or sitting position. Getting up slowly may help lessen the problem. These effects may also occur if you drink alcohol, stand for long periods of time, exercise, or if the weather is hot. While taking prazosin hydrochloride capsules, be careful in the amount of alcohol you drink. Also, use extra care during exercise or hot weather, or if standing for long periods. Check with your physician if you have any questions.
Prazosin hydrochloride capsules have 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 prazosin hydrochloride capsules have been shown to cause an additive hypotensive effect. This effect can be minimized by reducing the prazosin hydrochloride capsules dose to 1 mg to 2 mg three times a day, by introducing additional antihypertensive drugs cautiously, and then by retitrating prazosin hydrochloride capsules based on clinical response.
Concomitant administration of prazosin hydrochloride capsules with a phosphodiesterase-5 (PDE-5) inhibitor can result in additive blood pressure lowering effects and symptomatic hypotension (see DOSAGE AND ADMINISTRATION).
In a study on five patients given from 12 mg 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.
In clinical studies in which lipid profiles were followed, there were generally no adverse changes noted between pre- and post-treatment lipid levels.
No carcinogenic potential was demonstrated in an 18 month study in rats with prazosin hydrochloride capsules at dose levels more than 225 times the usual maximum recommended human dose of 20 mg per day. Prazosin hydrochloride capsules were not mutagenic in in vivo genetic toxicology studies. In a fertility and general reproductive performance study in rats, both males and females, treated with 75 mg/kg (225 times the usual maximum recommended human dose), demonstrated decreased fertility, while those treated with 25 mg/kg (75 times the usual maximum recommended human dose) did not.
In chronic studies (one year or more) of prazosin hydrochloride capsules in rats and dogs, testicular changes consisting of atrophy and necrosis occurred at 25 mg/kg/day (75 times the usual maximum recommended human dose). No testicular changes were seen in rats or dogs at 10 mg/kg/day (30 times the usual maximum recommended human dose). In view of the testicular changes observed in animals, 105 patients on long term prazosin hydrochloride capsules therapy were monitored for 17-ketosteroid excretion and no changes indicating a drug effect were observed. In addition, 27 males on prazosin hydrochloride capsules for up to 51 months did not have changes in sperm morphology suggestive of drug effect.
Prazosin hydrochloride capsules has been shown to be associated with decreased litter size at birth, 1, 4, and 21 days of age in rats when given doses more than 225 times the usual maximum recommended human dose. No evidence of drug-related external, visceral, or skeletal fetal abnormalities were observed. No drug-related external, visceral, or skeletal abnormalities were observed in fetuses of pregnant rabbits and pregnant monkeys at doses more than 225 times and 12 times the usual maximum recommended human dose, respectively.
The use of prazosin and a beta-blocker for the control of severe hypertension in 44 pregnant women revealed no drug-related fetal abnormalities or adverse effects. Therapy with prazosin was continued for as long as 14 weeks.1
Prazosin has also been used alone or in combination with other hypotensive agents in severe hypertension of pregnancy by other investigators. No fetal or neonatal abnormalities have been reported with the use of prazosin.2
There are no adequate and well controlled studies which establish the safety of prazosin hydrochloride capsules in pregnant women. Prazosin hydrochloride capsules should be used during pregnancy only if the potential benefit justifies the potential risk to the mother and fetus.
Prazosin has been shown to be excreted in small amounts in human milk. Caution should be exercised when prazosin hydrochloride capsules are administered to a nursing woman.
Safety and effectiveness in children have not been established.
Clinical trials were conducted on more than 900 patients. During these trials and subsequent marketing experience, the most frequent reactions associated with prazosin hydrochloride capsules 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% to 4% of patients are:
Gastrointestinal: vomiting, diarrhea, constipation.
Cardiovascular: edema, orthostatic hypotension, dyspnea, syncope.
Central Nervous System: vertigo, depression, nervousness.
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.
Central Nervous System: paresthesia, hallucinations.
Dermatologic: pruritus, alopecia, lichen planus.
Genitourinary: incontinence, impotence, priapism.
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 prazosin hydrochloride capsules 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.
Heart Rate/Rhythm: bradycardia.
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 (see PRECAUTIONS).
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