Classifications: alpha-adrenergic antagonist; antihypertensive; vasodilator;
; vasodilator
Pregnancy Category: C


1 mg, 2 mg, 5 mg capsules


Selective inhibition of alpha1-adrenoceptors; produces vasodilation in both resistance (arterioles) and capacitance (veins) vessels with the result that both peripheral vascular resistance and blood pressure are reduced.

Therapeutic Effect

Lowers blood pressure in supine and standing positions with most pronounced effect on diastolic pressure. Infrequently used in monotherapy because of tendency to support sodium and water retention resulting in increased plasma volume.


Treatment of hypertension.

Unlabeled Uses

Severe refractory congestive heart failure, Raynaud's disease or phenomenon, ergotamine-induced peripheral ischemia, pheochromocytoma, benign prostatic hypertrophy.


Safety during pregnancy (category C) is not established.

Cautious Use

Renal impairment; chronic kidney failure; hypertensive patient with cerebral thrombosis; angina; men with sickle cell trait; lactation.

Route & Dosage

Adult: PO Start with 1 mg h.s., then 1 mg b.i.d. or t.i.d., may increase to 20 mg/d in divided doses
Child: PO Start with 5 mcg/kg q6h, gradually increase to 25 mcg/kg q6h (max: 15 mg or 0.4 mg/kg/d)


  • Give initial dose at bedtime to reduce possibility of adverse effects such as postural hypotension and syncope. However, if first dose is taken during the day, advise patient not to drive a car for about 4 h after ingestion of drug.
  • Give drug with food to reduce incidence of faintness and dizziness; food may delay absorption but does not affect extent of absorption.
  • Store in tightly closed container away from strong light. Do not freeze.

Adverse Effects (≥1%)

CNS: Dizziness, headache, drowsiness, nervousness, vertigo, depression, paresthesia, insomnia. CV: Edema, dyspnea, syncope first-dose phenomenon, postural hypotension, palpitations, tachycardia, angina. Special Senses: Blurred vision, tinnitus, reddened sclerae. GI: Dry mouth, nausea, vomiting, diarrhea, constipation, abdominal discomfort, pain. Urogenital: Urinary frequency, incontinence, priapism (especially in men with sickle cell anemia), impotence. Skin: Rash, pruritus, alopecia, lichen planus. Body as a Whole: Diaphoresis, epistaxis, nasal congestion, arthralgia, transient leukopenia, increased serum uric acid, and BUN.


Drug: diuretics, hypotensive agents and alcohol increase hypotensive effects. Sildenafil, vardenafil, and tadalafil may enhance hypotensive effects.


Absorption: Approximately 60% of oral dose reaches the systemic circulation. Onset: 2 h. Peak: 2–4 h. Duration: <24 h. Distribution: Widely distributed, including into breast milk. Metabolism: Extensively in liver. Elimination: 6–10% in urine, rest in bile and feces. Half-Life: 2–4 h.

Nursing Implications

Assessment & Drug Effects

  • Be alert for first-dose phenomenon (rare adverse effect: 0.15% of patients); characterized by a precipitous decline in BP, bradycardia, and consciousness disturbances (syncope) within 90–120 min after the initial dose of prazosin. Recovery is usually within several hours. Preexisting low plasma volume (from diuretic therapy or salt restriction), beta-adrenergic therapy, and recent stroke appear to increase the risk of this phenomenon.
  • Monitor blood pressure. If it falls precipitously with first dose, notify physician promptly.
  • Full therapeutic effect may not be achieved until 4–6 wk of therapy.

Patient & Family Education

  • Avoid situations that would result in injury if you should faint, particularly during early phase of treatment. In most cases, effect does not recur after initial period of therapy; however, it may occur during acute febrile episodes, when drug dose is increased, or when another antihypertensive drug is added to the medication regimen.
  • Make position and direction changes slowly and in stages. Dangle legs and move ankles a minute or so before standing when arising in the morning or after a nap.
  • Lie down immediately if you experience light-headedness, dizziness, a sense of impending loss of consciousness, or blurred vision. Attempting to stand or walk may result in a fall.
  • Do not drive or engage in other potentially hazardous activities until response to drug is known.
  • Take drug at same time(s) each day. Keep a daily record noting BP and time taken, when medication was taken, which arm was used, position (i.e., standing, sitting), and time of day. Take this record to physician for reference at checkup appointment.
  • Report priapism or impotence. A change in the drug regimen usually reverses these difficulties. Since acute episodes of priapism followed by impotence spontaneously occur in men with sickle cell anemia, another antihypertensive should be selected. In these patients, drug-induced priapism is frequently irreversible.
  • Do not take OTC medications, especially those that may contain an adrenergic agent (e.g., remedies for coughs, colds, allergy), without consulting physician.
  • Be aware that adverse effects usually disappear with continuation of therapy, but dosage reduction may be necessary.

Common adverse effects in italic, life-threatening effects underlined; generic names in bold; classifications in SMALL CAPS; Canadian drug name; Prototype drug

© 2006-2023 Last Updated On: 01/30/2023 (0)
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