Prazosin

Trade Names:
Minipress
- Capsules 1 mg
- Capsules 2 mg
- Capsules 5 mg

Pharmacology

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Selectively blocks postsynaptic alpha-1 adrenergic receptors, resulting in dilation of arterioles and veins.

Pharmacokinetics

Absorption

T _max is approximately 3 h. Oral bioavailability is 48% to 68%.

Distribution

Protein binding is 92% to 97%.

Metabolism

Extensively metabolized by demethylation and conjugation in the liver.

Elimination

Urine (less than 10%); feces (less than 90%). Plasma t _½ is 2 to 3 h.

Duration

Duration of antihypertensive effect is 10 h.

Special Populations

Elimination is slower.

Elimination half-life prolonged, protein binding decreased, and peak plasma concentrations increased.

Indications and Usage

Treatment of hypertension.

Contraindications

Hypersensitivity to doxazosin, prazosin, or terazosin.

Dosage and Administration

PO 1 mg twice daily to 3 times daily.

PO 6 to 20 mg/day in divided doses (max, 40 mg/day).

PO 0.5 to 7 mg 3 times daily has been suggested.

General Advice

• Give initial dose at bedtime to avoid syncope.

Storage/Stability

Store at controlled room temperature (59° to 86°F).

Drug Interactions

Increased risk of hypotension.

Enhanced acute orthostatic hypotensive reaction after first dose of prazosin.

Increased serum prazosin levels and increased sensitivity to orthostatic hypotension.

Laboratory Test Interactions

May cause false elevation in vanillylmandelic acid.

Adverse Reactions

Cardiovascular

Palpitations; orthostatic hypotension; hypotension; tachycardia.

CNS

Depression; dizziness; nervousness; paresthesia; asthenia; drowsiness; headache.

Dermatologic

Pruritus; rash; sweating; alopecia; lichen planus.

EENT

Blurred vision; conjunctivitis; tinnitus; nasal congestion; epistaxis.

GI

Nausea; vomiting; dry mouth; diarrhea; constipation; abdominal discomfort or pain.

Genitourinary

Impotence; urinary frequency; incontinence; priapism.

Respiratory

Dyspnea.

Miscellaneous

Arthralgia; edema; fever.

Precautions

Pregnancy

Category C .

Lactation

Excreted in breast milk.

Children

Safety and efficacy not established.

Concomitant therapy

When adding a diuretic or other antihypertensive agent, reduce dosage to 1 to 2 mg 3 times daily and then retitrate.

First-dose effect

May cause marked hypotension (especially orthostatic) and syncope at 30 min after first few doses, after reintroduction, with rapid increase (at least 2 mg) in dosing, or after addition of another antihypertensive. To avoid, initiate dosing with low dose (1 mg or up to 2 mg) and gradually increase after 2 wk.

Lipids

May decrease total cholesterol levels and LDLs and increase HDLs.

Overdosage

Symptoms

Drowsiness, depressed reflexes, hypotension.

Patient Information

• Advise patient to take medication at same time each day.
• Warn patient about possibility of syncope or orthostasis.
• Instruct patient to report the following symptoms to health care provider: dizziness, palpitations, drowsiness, fatigue, nausea, headache.
• Caution patient to avoid sudden position changes to prevent orthostatic hypotension.
• Advise patient to avoid driving or performing other tasks requiring mental alertness for 12 to 24 h after first dose, after dosage increase, and after resuming treatment after interruption. After the 12- to 24-h period, advise patient to use caution.

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