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As with all alpha-blockers, MINIPRESS (prazosin hcl) may cause syncope with sudden loss of consciousness. In most cases, this is believed to be due to an excessive postural hypotensive effect, although occasionally the syncopal episode has been preceded by a bout of severe tachycardia with heart rates of 120-160 beats per minute. Syncopal episodes have usually occurred within 30 to 90 minutes of the initial dose of the drug; occasionally, they have been reported in association with rapid dosage increases or the introduction of another antihypertensive drug into the regimen of a patient taking high doses of MINIPRESS (prazosin hcl) . The incidence of syncopal episodes is approximately 1% in patients given an initial dose of 2 mg or greater. Clinical trials conducted during the investigational phase of this drug suggest that syncopal episodes can be minimized by limiting the initial dose of the drug to 1 mg, by subsequently increasing the dosage slowly, and by introducing any additional antihypertensive drugs into the patient's regimen with caution (See DOSAGE AND ADMINISTRATION). Hypotension may develop in patients given MINIPRESS (prazosin hcl) who are also receiving a beta-blocker such as propranolol.
If syncope occurs, the patient should be placed in the recumbent position and treated supportively as necessary. This adverse effect is self-limiting and in most cases does not recur after the initial period of therapy or during subsequent dose titration.
Patients should always be started on the 1 mg capsules of MINIPRESS (prazosin hcl) . The 2 and 5 mg capsules are not indicated for initial therapy.
More common than loss of consciousness are the symptoms often associated with lowering of the blood pressure, namely, dizziness and lightheadedness. The patient should be cautioned about these possible adverse effects and advised what measures to take should they develop. The patient should also be cautioned to avoid situations where injury could result should syncope occur during the initiation of MINIPRESS (prazosin hcl) therapy.
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.
Carcinogenesis, Mutagenesis, Impairment of Fertility
No carcinogenic potential was demonstrated in an 18 month study in rats with MINIPRESS (prazosin hcl) at dose levels more than 225 times the usual maximum recommended human dose of 20 mg per day. MINIPRESS (prazosin hcl) was 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 MINIPRESS (prazosin hcl) 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 MINIPRESS (prazosin hcl) therapy were monitored for 17-ketosteroid excretion and no changes indicating a drug effect were observed. In addition, 27 males on MINIPRESS (prazosin hcl) for up to 51 months did not have changes in sperm morphology suggestive of drug effect.
Usage in Pregnancy
Pregnancy Category C. MINIPRESS (prazosin hcl) 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 MINIPRESS (prazosin hcl) in pregnant women. MINIPRESS (prazosin hcl) should be used during pregnancy only if the potential benefit justifies the potential risk to the mother and fetus.
MINIPRESS (prazosin hcl) has been shown to be excreted in small amounts in human milk. Caution should be exercised when MINIPRESS (prazosin hcl) is administered to a nursing woman.
Usage in Children
Safety and effectiveness in children have not been established.
1. Lubbe, WF, and Hodge, JV: New Zealand Med J, 94 (691) 169-172, 1981.
2. Davey, DA, and Dommisse, J: S.A. Med J, Oct. 4, 1980 (551-556).
Last reviewed on RxList: 8/4/2009
This monograph has been modified to include the generic and brand name in many instances.
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