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Prazosin (Systemic)


VA CLASSIFICATION
Primary: CV150
Secondary: CV409; CV900; AD900; GU900

Commonly used brand name(s): Minipress.

Note: For a listing of dosage forms and brand names by country availability, see Dosage Forms section(s).



Category:


Antihypertensive—

vasodilator, congestive heart failure—

antidote (to ergot alkaloid poisoning)—

vasospastic therapy adjunct—

benign prostatic hyperplasia therapy agent—

Indications

Note: Bracketed information in the Indications section refers to uses that are not included in U.S. product labeling.

Accepted

Hypertension (treatment)—Prazosin is indicated in the treatment of hypertension . {01} {02} {03} {58} {83} {123} {124}
—For additional information on initial therapeutic guidelines related to the treatment of hypertension, see Appendix III.

[Congestive heart failure (treatment) ]1—Prazosin may be used as an adjunct to digoxin and diuretics for the treatment of congestive heart failure. {04} {05} {06} {07} {08} {09} {10} {11} {12} {61} However, prazosin has not been shown to improve survival in these patients. {61}

[Toxicity, ergot alkaloid (treatment) ]1—Prazosin is used for treatment of peripheral vasospasm caused by ergot alkaloid overdose. {13} {74}

[Pheochromocytoma (treatment)]1—Prazosin is used for the management of hypertension associated with pheochromocytoma. {75} {76} {77} {78} {79} {80}

[Raynaud"s phenomenon (treatment) ]1—Prazosin is used for treatment of Raynaud"s phenomenon. {14} {41} {48} {49} {50}

[Benign prostatic hyperplasia (BPH) (treatment)]1—Prazosin is used for the treatment of urinary symptoms associated with benign prostatic hyperplasia. {62} {63} {64} {65} {66} {67} {68} {69} {70} {71} {72} {73} Prazosin has been shown to improve urinary flow and symptoms of BPH. {62} {66} However, the long-term effects of prazosin on the incidence of acute urinary obstruction or other complications of BPH or on the need for surgery have not yet been determined.

1 Not included in Canadian product labeling.



Pharmacology/Pharmacokinetics

Physicochemical characteristics:
Molecular weight—
    419.87 {51}

pKa—
    6.5 {29} {81}

Mechanism of action/Effect:

Prazosin is a selective alpha 1-adrenergic blocking agent. The alpha 1-adrenergic blocking action is thought to account primarily for its effects. {01} {16} {17} {18} {20} {21} {22} {23}


Hypertension:

Prazosin produces vasodilation and reduces peripheral resistance but generally has little effect on cardiac output. {01} {03} {18} {21} {22} {23} {24} {25} Antihypertensive effect is usually not accompanied by reflex tachycardia. {01} {22} {25} {60} There is little or no effect on renal blood flow or glomerular filtration rate. {01} {28} {29} {58}



Congestive heart failure:

Beneficial effects, resulting from vasodilation, are due to decreased systemic resistance, preload and afterload reduction, and resulting improved cardiac output. {04} {05} {09} {10}



Raynaud's phenomenon:

Therapeutic effect for vasospasm is due to inhibition of vasoconstriction by blocking of postsynaptic alpha 1 receptors. {48} {49}



Benign prostatic hyperplasia:

Relaxation of smooth muscle in the bladder neck, prostate, and prostate capsule produced by alpha 1-adrenergic blockade results in a reduction in urethral resistance and pressure, bladder outlet resistance, and urinary symptoms. {69} {121}



Other actions/effects:

Prazosin may affect serum lipids. The most consistent changes observed are a decrease in levels of serum total cholesterol and low density lipoprotein (LDL) cholesterol. {52} {53} {54} {55} {56} {59} However, the implications of these changes are unclear. {120}

Absorption:

Well-absorbed from gastrointestinal tract; bioavailability is variable (50 to 85%). {01} {22} {27} {28} {60}

Protein binding:

Very high {28} {29} (97% {28} {29} {43} {83}; 20% to red blood cells {29}).

Biotransformation:

Primarily hepatic. {43} Several metabolites have been identified in humans and animals (6-O-demethyl, 7-O-demethyl, 2-[1-piperazinyl]-4-amino-6, 7-dimethoxyquinazoline, 2,4-diamino-6,7-dimethoxyquinazoline); {35} {42} {43} in dog studies, three of the metabolites were shown to be responsible for approximately 10 to 25% of prazosin's hypotensive activity. {35}

Half-life:

2 to 3 hours {01} {16} {22} {28} {30} {31} {32} {33} {43} {58} {81}; unchanged in renal function impairment {31}, but may increase to more than double (6 to 8 hours) in congestive heart failure. {07} {22} {32}

Onset of action:

Hypertension—Within 30 to 90 minutes after a single dose. {58}

Congestive heart failure—Rapid. {05}

Time to peak concentration:

1 to 3 hours. {22} {30} {33} {39} {43} {58} {60}

Time to peak effect:


Hypertension:

Single dose: 2 to 4 hours. {16} {28} {30} {32}

Multiple doses: Up to 3 to 4 weeks of therapy may be required for maximal therapeutic effect.



Congestive heart failure:

1 hour. {05}


Duration of action:

Hypertension—Single dose: 7 to 10 hours {33} {119}.

Congestive heart failure—6 hours. {05}

Elimination:
    Primarily in bile and feces; 6 to 10% in urine. Excreted as unchanged drug (5 to 11%) and metabolites. {27} {28} {35} {81} Elimination of prazosin may be slower in patients with congestive heart failure than in normal subjects. {32}
    In dialysis—Not dialyzable. {58}


Precautions to Consider

Cross-sensitivity and/or related problems

Patients sensitive to other quinazolines (doxazosin, terazosin) may also be sensitive to prazosin. {83}

Carcinogenicity

An 18-month study in rats given prazosin at doses of more than 225 times the usual maximum recommended human dose of 20 mg per day did not demonstrate carcinogenic potential. {58}

Mutagenicity

Prazosin was not mutagenic in in vitro genetic toxicology studies. {58}

Pregnancy/Reproduction
Fertility—
A study in male rats given subcutaneous injections of prazosin (1.4 mg per kg of body weight [mg/kg]) revealed reduced fertility manifested by a suppression of the fertilizing potential of spermatozoa. {84} {122} {123}

A fertility and general reproductive performance study in male and female rats given prazosin at a dose of 75 mg/kg (225 times the usual maximum recommended human dose) demonstrated decreased fertility. However, when rats were given 25 mg/kg (75 times the usual maximum recommended human dose) decreased fertility was not seen. {58} {123}

Pregnancy—
Adequate and well-controlled studies in humans have not been done. However, limited uncontrolled use of prazosin and a beta-blocking agent for the control of severe hypertension in 44 pregnant women revealed no drug-related fetal abnormalities or adverse effects. {58} {92} Also, use of prazosin during the last trimester in 8 pregnant women with hypertension produced no prolonged clinical problems. All infants were developing normally 6 to 30 months following delivery. {93}

In rats given doses more than 225 times the usual maximum recommended human dose, prazosin has been shown to be associated with decreased litter weight at birth and at 1, 4, and 21 days of age. There was no evidence, however, of drug-related external, visceral, or skeletal abnormalities. {58} {122}

In pregnant rabbits and pregnant monkeys given doses of more than 225 times and 12 times the usual maximum recommended human dose, respectively, no drug-related external, visceral, or skeletal abnormalities were observed in the fetuses. {58}

FDA Pregnancy Category C. {58} {122}

Breast-feeding

Prazosin is distributed into breast milk in small amounts. {58} {83} {123}

Pediatrics

Appropriate studies on the relationship of age to the effects of prazosin have not been performed in the pediatric population. However, no pediatrics-specific problems have been documented to date.


Geriatrics


Although appropriate studies on the relationship of age to the effects of prazosin have not been performed in the geriatric population, geriatrics-specific problems are not expected to limit the usefulness of prazosin in the elderly. However, elderly patients may be more sensitive to the hypotensive effects and are more likely to have age-related renal function impairment, which may require lower prazosin doses. In addition, the risk of prazosin-induced hypothermia may be increased in elderly patients.

Drug interactions and/or related problems
The following drug interactions and/or related problems have been selected on the basis of their potential clinical significance (possible mechanism in parentheses where appropriate)—not necessarily inclusive (» = major clinical significance):


Note: Combinations containing any of the following medications, depending on the amount present, may also interact with this medication.

Anti-inflammatory drugs, nonsteroidal (NSAIDs), especially indomethacin{40}{85}{86}{87}{88}    (antihypertensive effects of prazosin may be reduced when it is used concurrently with these agents; indomethacin, and possibly other NSAIDs, may antagonize the antihypertensive effect by inhibiting renal prostaglandin synthesis and/or by causing sodium and fluid retention; the patient should be carefully monitored to confirm that the desired effect is being obtained)


Hypotension-producing medications, other (see Appendix II )    (antihypertensive effects may be potentiated when these medications are used concurrently with prazosin; although some antihypertensive and/or diuretic combinations are frequently used to therapeutic advantage, dosage adjustments are necessary when these medications are used concurrently)


Sympathomimetics    (antihypertensive effects of prazosin may be reduced when it is used concurrently with these agents; the patient should be carefully monitored to confirm that the desired effect is being obtained)

    (concurrent use of prazosin antagonizes the peripheral vasoconstriction produced by high doses of dopamine)

    (concurrent use of prazosin may decrease the pressor response to ephedrine)

    (concurrent use of prazosin may block the alpha-adrenergic effects of epinephrine, possibly resulting in severe hypotension and tachycardia)

    (concurrent use of prazosin usually decreases, but does not reverse or completely block, the pressor effect of metaraminol)

    (prior administration of prazosin may decrease the pressor effect and shorten the duration of action of methoxamine and phenylephrine)



Laboratory value alterations
The following have been selected on the basis of their potential clinical significance (possible effect in parentheses where appropriate)—not necessarily inclusive (» = major clinical significance):

With diagnostic test results
Vanillylmandelic acid (VMA), urinary    (concentrations may be increased; false positive results may occur in screening tests for pheochromocytoma {58} {83} {91})


Medical considerations/Contraindications
The medical considerations/contraindications included have been selected on the basis of their potential clinical significance (reasons given in parentheses where appropriate)— not necessarily inclusive (» = major clinical significance).


Risk-benefit should be considered when the following medical problems exist
Angina pectoris{47}{90}    (may induce angina or aggravate pre-existing angina)


» Cardiac disease, severe{90}    (prazosin is usually not used alone, although it may improve cardiac performance in some patients with severe refractory congestive heart failure)


Narcolepsy{105}    (may exacerbate cataplexy; however, a clear cause-effect relationship has not been established)


Renal function impairment    (increased sensitivity to prazosin's effects; lower doses may be required {31} {44} {45} {46} {83} {89})


Sensitivity to prazosin

Patient monitoring
The following may be especially important in patient monitoring (other tests may be warranted in some patients, depending on condition; » = major clinical significance):

» Blood pressure measurements    (recommended at periodic intervals in patients being treated for hypertension; selected patients may be trained to perform blood pressure measurements at home and report the results at regular physician visits)




Side/Adverse Effects

Note: A “first-dose orthostatic hypotensive reaction” sometimes occurs, most frequently 30 to 90 minutes after the initial dose of prazosin, and may be severe. Syncope or other postural symptoms, such as dizziness, may occur. Subsequent occurrence with dosage increases is also possible. Incidence appears to be dose-related; thus, it is important that therapy be initiated with the lowest possible dose. Patients who are volume-depleted or sodium-restricted may be more sensitive to the orthostatic hypotensive effects of prazosin, and the effect may be exaggerated after exercise. {01} {58} {83}
Hypotensive side effects may be more likely to occur in geriatric patients.

The following side/adverse effects have been selected on the basis of their potential clinical significance (possible signs and symptoms in parentheses where appropriate)—not necessarily inclusive:

Those indicating need for medical attention
Incidence more frequent
    
Dizziness{58}{83}{94}{95}
    
orthostatic hypotension (dizziness or lightheadedness when getting up from a lying or sitting position; sudden fainting){83}{94}

Incidence less frequent
    
Edema (swelling of feet or lower legs){58}{94}{95}
    
palpitations (pounding heartbeat){58}{83}{94}{95}
    
urinary incontinence (loss of bladder control){58}{83}{110}{111}{112}{113}

Incidence rare
    
Angina (chest pain){47}{95}{106}
    
dyspnea (shortness of breath){58}{95}
    
priapism (painful, inappropriate erection of the penis, continuing){58}{83}{114}{115}{116}{117}



Those indicating need for medical attention only if they continue or are bothersome
Incidence more frequent
    
Drowsiness{58}{83}{94}{95}
    
headache{58}{83}{94}{95}
    
malaise (lack of energy){58}{83}{94}{95}

Incidence less frequent
    
Dryness of mouth{58}{83}{94}{95}
    
fatigue (unusual tiredness or weakness){58}{83}{95}
    
nervousness{58}{94}{95}

Incidence rare
    
Nausea{58}{83}{95}
    
urinary frequency (frequent urge to urinate){58}{95}





Overdose
For more information on the management of overdose or unintentional ingestion, contact a Poison Control Center (see Poison Control Center Listing ).

Treatment of overdose
Recommended treatment for prazosin overdose includes: Treatment of circulatory failure, either by placing the patient in the supine position and elevating the legs or by using additional measures if shock is present, is most important; volume expanders may be used to treat shock, followed, if necessary, by administration of a vasopressor {01} {58}; symptomatic, supportive treatment and monitoring of fluid and electrolyte status {01} {58}.


Patient Consultation
As an aid to patient consultation, refer to Advice for the Patient, Prazosin (Systemic).

In providing consultation, consider emphasizing the following selected information (» = major clinical significance):

Before using this medication
»   Conditions affecting use, especially:
Sensitivity to quinazolines





Breast-feeding—Distributed into breast milk in small amounts





Use in the elderly—Increased sensitivity to hypotensive effects and increased risk of prazosin-induced hypothermia
Other medical problems, especially severe cardiac disease

Proper use of this medication
Compliance with therapy; taking medication at the same times each day to maintain the therapeutic effect

» Proper dosing
Missed dose: Taking as soon as possible; not taking if almost time for next dose; not doubling doses

» Proper storage

For use as an antihypertensive
Possible need for control of weight and diet, especially sodium intake {120}

» Patient may not experience symptoms of hypertension; importance of taking medication even if feeling well

» Does not cure, but helps control hypertension; possible need for lifelong therapy; serious consequences of untreated hypertension

For use in benign prostatic hyperplasia (BPH)
Relieves symptoms of BPH but does not change the size of the prostate; may not prevent the need for surgery in the future

Precautions while using this medication
Making regular visits to physician to check progress

» Caution if dizziness, lightheadedness, or sudden fainting occurs, especially after initial dose; taking first dose at bedtime

» Caution when getting up suddenly from a lying or sitting position

» Caution in using alcohol, while standing for long periods or exercising, and during hot weather because of enhanced orthostatic hypotensive effects

» Possibility of drowsiness

» Caution when driving or doing anything else requiring alertness because of possible drowsiness, dizziness, or lightheadedness

» Not taking other medications, especially nonprescription sympathomimetics, unless discussed with physician


Side/adverse effects
Signs of potential side effects, especially dizziness, orthostatic hypotension, edema, palpitations, urinary incontinence, angina, dyspnea, and priapism


General Dosing Information
Dosage of prazosin should be adjusted to meet the individual requirements of each patient, on the basis of blood pressure response.

Prazosin may be used alone or in combination with a thiazide diuretic or beta-adrenergic blocker {58}, both of which reduce the tendency for sodium and water retention, although they also produce additive hypotension. If combination therapy is indicated, individual titration is required to ensure the lowest possible therapeutic dose of each drug.

In order to minimize the “first-dose orthostatic hypotensive reaction,” an initial dose of 1 mg {58} is recommended, with gradual increments as needed. {01} {58} {83} Administration of the initial dose at bedtime is recommended, as well as the initial dose of each increment.

When a diuretic or other antihypertensive agent is added to prazosin therapy, the dose of prazosin should be reduced to 1 or 2 mg three times a day, followed by titration of dosage of the combination. {58} When prazosin is added to existing diuretic or antihypertensive therapy, the dose of the other agent should be reduced {83} and prazosin started at a dose of 0.5 {83} or 1 mg two or three times a day.

Tolerance to the effects of prazosin may occur during treatment of congestive heart failure but usually not during treatment of hypertension. An early, transient (usually within the first few doses) blunting of hemodynamic effect may occur due to reflex activation of the sympathetic nervous system. {05} {07} {08} {96} The hemodynamic effect may spontaneously restore with uninterrupted therapy {05} or the blunted effect may be overcome by temporarily interrupting prazosin therapy. {08} A later apparent tolerance {05} {97} {98} may result from fluid retention, requiring increased doses of diuretics {04} {05}; this effect may be minimized by increasing the dose of prazosin {05}, temporarily interrupting prazosin therapy {05}, or substituting another vasodilator.


Oral Dosage Forms

Note: Bracketed uses in the Dosage Forms section refer to categories of use and/or indications that are not included in U.S. product labeling.

PRAZOSIN HYDROCHLORIDE CAPSULES USP

Note: The dosing and strengths of the dosage forms available are expressed in terms of prazosin base (not the hydrochloride salt).


Usual adult dose
Antihypertensive
Initial: Oral, 1 mg (base) two or three times a day. {01} {58}

Maintenance: Oral, adjusted gradually to meet individual requirements, most commonly 6 to 15 mg (base) a day in two or three divided doses. {01} {58}

[Toxicity, ergot alkaloid]
Oral, 1 mg three times a day. {13} {74}

[Vasospastic therapy adjunct—Raynaud's phenomenon]
Oral, 1 mg three times a day. {41} {48} {49}

[Benign prostatic hyperplasia]
Initial: Oral, 1 mg (base) two times a day. {62} {63} {64} {65} {66} {67} {68} {69} {70} {71} {72} {73} {118}

Maintenance: Oral, 1 to 5 mg (base) two times a day. {62} {63} {64} {65} {66} {67} {68} {69} {70} {71} {72} {73} {118}


Note: Geriatric patients may be more sensitive to the effects of the usual adult dose.


Usual adult prescribing limits
Daily doses higher than 20 mg (base) usually do not have increased efficacy, although some patients respond to up to 40 mg a day. {01} {58}

Usual pediatric dose
Antihypertensive
Oral, 50 to 400 mcg (0.05 to 0.4 mg) (base) per kg of body weight per day {127} in two or three divided doses {128} {130}. Single doses should not exceed 7 mg, {128} and the total daily dose should not exceed 15 mg per day {129}.


Strength(s) usually available
U.S.—


1 mg (base) (Rx) [Minipress (sucrose)][Generic](sucrose)


2 mg (base) (Rx) [Minipress (sucrose)][Generic](sucrose)


5 mg (base) (Rx) [Minipress (sucrose)][Generic](sucrose)

Canada—
Not commercially available.

Packaging and storage:
Store below 40 °C (104 °F), preferably between 15 and 30 °C (59 and 86 °F), unless otherwise specified by manufacturer. Store in a well-closed, light-resistant container. {99}

Auxiliary labeling:
   • Do not take other medicines without your doctor's advice.
   • May cause dizziness.

Note: Check refill frequency to determine compliance in hypertensive patients.



PRAZOSIN HYDROCHLORIDE TABLETS

Note: The dosing and strengths of the dosage forms available are expressed in terms of prazosin base (not the hydrochloride salt).


Usual adult dose
Antihypertensive
Initial: Oral, 500 mcg (0.5 mg) two or three times a day for at least 3 days. If tolerated, increase to 1 mg (base) two or three times a day for a further 3 days.

Maintenance: Oral, adjusted gradually to meet individual requirements, most commonly 6 to 15 mg (base) a day in two or three divided daily doses.

Toxicity, ergot alkaloid1
Oral, 1 mg three times a day. {13} {74}

Vasospastic therapy adjunct—Raynaud's phenomenon1
Oral, 1 mg three times a day. {41} {48} {49}

Benign prostatic hyperplasia1
Initial: Oral, 1 mg (base) two times a day. {62} {63} {64} {65} {66} {67} {68} {69} {70} {71} {72} {73} {118}

Maintenance: Oral, 1 to 5 mg (base) two times a day. {62} {63} {64} {65} {66} {67} {68} {69} {70} {71} {72} {73} {118}


Note: Geriatric patients may be more sensitive to the effects of the usual adult dose.


Usual adult prescribing limits
Daily doses higher than 20 mg (base) usually do not have increased efficacy, although some patients respond to up to 40 mg a day.

Usual pediatric dose
Antihypertensive
See Prazosin Hydrochloride Capsules USP .


Strength(s) usually available
U.S.—
Not commercially available.

Canada—


1 mg (base) (Rx) [Minipress (scored)][Generic]


2 mg (base) (Rx) [Minipress (scored)][Generic]


5 mg (base) (Rx) [Minipress (scored)][Generic]

Packaging and storage:
Store below 40 °C (104 °F), preferably between 15 and 30 °C (59 and 86 °F), unless otherwise specified by manufacturer. Store in a well-closed, light-resistant container.

Auxiliary labeling:
   • Do not take other medicines without your doctor's advice.
   • May cause dizziness.

Note: Check refill frequency to determine compliance in hypertensive patients.




Revised: 08/19/1998



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  1. Urology panel meeting, 11/25/91.
  1. Open.
  1. Cardiology and Renal Drugs panel meeting, 2/25/92.
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  1. Prazosin hydrochloride capsules package insert (Zenith—US), Rev 3/92, Rec 8/93.
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  1. Panel comment, 3/92.
  1. Panel comment, 3/92
  1. Panel comment, 3/92.
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