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Prazosin

Class: alpha-Adrenergic Blocking Agents
VA Class: CV150
CAS Number: 19237-84-4
Brands: Minipress

Medically reviewed by Drugs.com. Last updated on March 29, 2021.

Introduction

Postsynaptic α1-adrenergic blocking agent; quinazoline derivative.

Uses for Prazosin

Hypertension

Management of hypertension (alone or in combination with other classes of antihypertensive agents).

Not considered a preferred agent for initial management of hypertension according to current evidence-based hypertension guidelines, but may be useful in the management of resistant hypertension as a component of combination therapy.

Most effective when used in combination with a diuretic; beneficial effects of α1-blockers on blood glucose and lipid concentrations also may mitigate some adverse metabolic effects of diuretics.

Some experts state that an α1-blocker may be a second-line agent in antihypertensive treatment regimens in men with coexisting benign prostatic hyperplasia (BPH); however, the American Urology Association (AUA) states that monotherapy with these drugs is not optimal in hypertensive patients with lower urinary tract symptoms (LUTS) or BPH and that such conditions should be managed separately.

Individualize choice of therapy; consider patient characteristics (e.g., age, ethnicity/race, comorbidities, cardiovascular risk) as well as drug-related factors (e.g., ease of administration, availability, adverse effects, cost).

A 2017 ACC/AHA multidisciplinary hypertension guideline classifies BP in adults into 4 categories: normal, elevated, stage 1 hypertension, and stage 2 hypertension. (See Table 1.)

Source: Whelton PK, Carey RM, Aronow WS et al. 2017 ACC/AHA/AAPA/ABC/ACPM/AGS/APhA/ASH/ASPC/NMA/PCNA guideline for the prevention, detection, evaluation, and management of high blood pressure in adults: a report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines. Hypertension. 2018;71:e13-115.

Individuals with SBP and DBP in 2 different categories (e.g., elevated SBP and normal DBP) should be designated as being in the higher BP category (i.e., elevated BP).

Table 1. ACC/AHA BP Classification in Adults1200

Category

SBP (mm Hg)

DBP (mm Hg)

Normal

<120

and

<80

Elevated

120–129

and

<80

Hypertension, Stage 1

130–139

or

80–89

Hypertension, Stage 2

≥140

or

≥90

The goal of hypertension management and prevention is to achieve and maintain optimal control of BP. However, the BP thresholds used to define hypertension, the optimum BP threshold at which to initiate antihypertensive drug therapy, and the ideal target BP values remain controversial.

The 2017 ACC/AHA hypertension guideline generally recommends a target BP goal (i.e., BP to achieve with drug therapy and/or nonpharmacologic intervention) of <130/80 mm Hg in all adults regardless of comorbidities or level of atherosclerotic cardiovascular disease (ASCVD) risk. In addition, an SBP goal of <130 mm Hg generally is recommended for noninstitutionalized ambulatory patients ≥65 years of age with an average SBP of ≥130 mm Hg. These BP goals are based upon clinical studies demonstrating continuing reduction of cardiovascular risk at progressively lower levels of SBP.

Other hypertension guidelines generally have based target BP goals on age and comorbidities. Guidelines such as those issued by the JNC 8 expert panel generally have targeted a BP goal of <140/90 mm Hg regardless of cardiovascular risk, and have used higher BP thresholds and target BPs in elderly patients compared with those recommended by the 2017 ACC/AHA hypertension guideline.

Some clinicians continue to support previous target BPs recommended by JNC 8 due to concerns about the lack of generalizability of data from some clinical trials (e.g., SPRINT study) used to support the 2017 ACC/AHA hypertension guideline and potential harms (e.g., adverse drug effects, costs of therapy) versus benefits of BP lowering in patients at lower risk of cardiovascular disease.

Consider potential benefits of hypertension management and drug cost, adverse effects, and risks associated with the use of multiple antihypertensive drugs when deciding a patient's BP treatment goal.

For decisions regarding when to initiate drug therapy (BP threshold), the 2017 ACC/AHA hypertension guideline incorporates underlying cardiovascular risk factors. ASCVD risk assessment is recommended by ACC/AHA for all adults with hypertension.

ACC/AHA currently recommend initiation of antihypertensive drug therapy in addition to lifestyle/behavioral modifications at an SBP ≥140 mm Hg or DBP ≥90 mm Hg in adults who have no history of cardiovascular disease (i.e., primary prevention) and a low ASCVD risk (10-year risk <10%).

For secondary prevention in adults with known cardiovascular disease or for primary prevention in those at higher risk for ASCVD (10-year risk ≥10%), ACC/AHA recommend initiation of antihypertensive drug therapy at an average SBP ≥130 mm Hg or an average DBP ≥80 mm Hg.

Adults with hypertension and diabetes mellitus, chronic kidney disease (CKD), or age ≥65 years are assumed to be at high risk for cardiovascular disease; ACC/AHA state that such patients should have antihypertensive drug therapy initiated at a BP ≥130/80 mm Hg. Individualize drug therapy in patients with hypertension and underlying cardiovascular or other risk factors.

In stage 1 hypertension, experts state that it is reasonable to initiate drug therapy using the stepped-care approach in which one drug is initiated and titrated and other drugs are added sequentially to achieve the target BP. Initiation of antihypertensive therapy with 2 first-line agents from different pharmacologic classes recommended in adults with stage 2 hypertension and average BP >20/10 mm Hg above BP goal.

Benign Prostatic Hyperplasia

Has been used to reduce urinary obstruction and relieve associated manifestations in patients with symptomatic BPH; efficacy relative to other α1-adrenergic blockers remains to be established.

Posttraumatic Stress Disorder (PTSD)

Has been used in the management of PTSD, particularly in combat veterans and in patients experiencing nighttime PTSD symptoms (e.g., nightmares, sleep disturbances).

Some clinicians recommend prazosin as first-line or alternative therapy when treating PTSD patients with prominent nighttime symptoms, particularly in combat veterans. Further studies necessary in civilians with noncombat trauma-related PTSD and in treatment of daytime PTSD symptoms.

Prazosin Dosage and Administration

General

BP Monitoring and Treatment Goals

  • Monitor BP regularly (i.e., monthly) during therapy and adjust dosage of the antihypertensive drug until BP controlled.

  • If unacceptable adverse effects occur, discontinue drug and initiate another antihypertensive agent from a different pharmacologic class.

  • If adequate BP response not achieved with a single antihypertensive agent, either increase dosage of single drug or add a second drug with demonstrated benefit and preferably a complementary mechanism of action (e.g., ACE inhibitor, angiotensin II receptor antagonist, calcium-channel blocker, thiazide diuretic). Many patients will require ≥2 drugs from different pharmacologic classes to achieve BP goal; if goal BP still not achieved with 2 antihypertensive agents, add a third drug.

Administration

Oral Administration

Administer orally in divided doses 2 or 3 times daily.

Manufacturers make no specific recommendations regarding administration with meals.

Dosage

Available as prazosin hydrochloride; dosage expressed in terms of prazosin.

Individualize dosage according to patient response and tolerance. Initiate at low dosage to minimize frequency of postural hypotension and syncope.

Postural effects are most likely to occur 2–6 hours after a dose; monitor BP during this period after first dose and with any dosage increases.

If therapy is interrupted for a few days, restart using initial dosage regimen.

Pediatric Patients

Hypertension†
Oral

Some experts have recommended an initial dosage of 0.05–0.1 mg/kg daily given in 3 divided doses. Increase dosage as necessary up to a maximum of 0.5 mg/kg daily given in 3 divided doses. (See Pediatric Use under Cautions.)

Adults

Hypertension
Oral

Initially, 1 mg 2 or 3 times daily. Do not initiate with higher dosages. May increase dosage gradually to 20 mg daily given in divided doses.

Usual maintenance dosage: Manufacturer states 6–15 mg daily given in divided doses. Some experts state 2–20 mg daily, administered in 2 or 3 divided doses.

When other hypotensive agents or diuretics are added to existing prazosin therapy, reduce dosage to 1 or 2 mg 3 times daily; gradually increase according to patient's response and tolerance.

Posttraumatic Stress Disorder†
Oral

Optimum dosage not established. In clinical studies, usual initial dosage was 1 mg at bedtime; dosage was then gradually increased based on patient's response and tolerance. Maintenance dosages of 1–25 mg daily (given once daily at bedtime or in 2 divided doses) have been used. Some experts recommend a target maintenance dosage of 1–10 mg daily; others recommend a higher target dosage of 2–20 mg daily.

Prescribing Limits

Pediatric Patients

Hypertension†
Oral

Maximum 0.5 mg/kg daily.

Adults

Hypertension
Oral

Maximum 20 mg daily. Although higher dosages usually do not increase efficacy, a few patients may benefit from ≤40 mg daily.

Special Populations

Hepatic Impairment

No specific dosage recommendations at this time.

Renal Impairment

Initially, 1 mg twice daily. Patients with chronic renal failure may require only small dosages.

Geriatric Patients

No specific dosage recommendations at this time; generally increase dosage more slowly in geriatric hypertensive patients than in younger adults.

Cautions for Prazosin

Contraindications

Known hypersensitivity to prazosin, quinazolines (e.g., alfuzosin, doxazosin, terazosin), or any ingredient in the formulation.

Warnings/Precautions

Warnings

Postural Hypotension

Like other α-adrenergic blocking agents, marked hypotension, especially in the upright position, can occur; may be accompanied by syncope, palpitations, and other postural effects (e.g., dizziness, lightheadedness, vertigo).

Postural effects are most common after an initial dose, shortly after dosing (e.g., within 90 minutes), when dosage is rapidly increased, or when other antihypertensive agents are added to therapy.

To decrease risk of excessive hypotension and syncope, initiate therapy at a low dosage (i.e., 1 mg) and titrate slowly; initiate concomitant antihypertensive agents with caution.

If syncope or hypotension occurs, place patient in a recumbent position and institute supportive therapy as necessary.

General Precautions

Intraoperative Floppy Iris Syndrome (IFIS)

IFIS observed during cataract surgery in some patients currently receiving or previously treated with α1-adrenergic blocking agents.

If patient has received α1-blockers, ophthalmologist should be prepared to modify the surgical technique (e.g., through use of iris hooks, iris dilator rings, viscoelastic substances) to minimize complications of IFIS.

Benefit of discontinuing α1-blockers, including prazosin, prior to cataract surgery not established.

Prostate Cancer

Exclude possibility of prostate cancer before initiation of therapy for BPH.

Specific Populations

Pregnancy

Category C.

Lactation

Distributed into milk in small amounts. Caution if used in nursing women.

Pediatric Use

Manufacturer states that safety and efficacy not established in children.

Some experts suggest reserving use of centrally acting antihypertensive agents (e.g., prazosin) for children who do not respond to therapy with 2 or more preferred classes of antihypertensive agents (angiotensin-converting enzyme [ACE] inhibitors, angiotensin II receptor antagonists, long-acting calcium-channel blockers, or thiazide diuretics).

Geriatric Use

Geriatric patients may be particularly susceptible to postural effects and other adverse effects.

Common Adverse Effects

Dizziness, lightheadedness, headache, drowsiness, lack of energy, weakness, palpitation, nausea.

Interactions for Prazosin

Protein-bound Drugs

Potential pharmacokinetic interaction (displacement of prazosin or other protein-bound drug).

Specific Drugs and Laboratory Tests

Drug

Interaction

Comments

Analgesic agents (aspirin, indomethacin)

No interaction observed

Antiarrhythmic agents (procainamide, quinidine)

No interaction observed

Antigout agents (allopurinol, colchicine, probenecid)

No interaction observed

Antihypertensive agents (e.g., propranolol)

Possible additive hypotensive effects and symptomatic hypotension

Initiate additional antihypertensive agents with caution; may reduce prazosin dosage and gradually increase dosage based on clinical response

Benzodiazepines (chlordiazepoxide, diazepam)

No interaction observed

Digoxin

No interaction observed

Diuretics

Possible additive hypotensive effects and symptomatic hypotension

Effect usually used to therapeutic advantage

Initiate diuretics with caution; may reduce prazosin dosage and gradually increase dosage based on clinical response

Hypoglycemic agents (insulin, chlorpropamide, phenformin [no longer commercially available in the US], tolazamide, tolbutamide)

No interaction observed

Phenobarbital

No interaction observed

Phosphodiesterase (PDE) type 5 inhibitors (e.g., sildenafil, tadalafil, vardenafil)

Possible additive hypotensive effects and symptomatic hypotension

Initiate PDE type 5 inhibitor at lowest possible dosage

Test for pheochromocytoma

Possible increase in urinary metabolite of norepinephrine and VMA; false positive results may occur in pheochromocytoma screening tests

If elevated VMA is observed, discontinue prazosin and repeat test after 1 month

Prazosin Pharmacokinetics

Absorption

Bioavailability

Peak plasma concentrations attained within about 2–3 hours.

Bioavailability is approximately 60%.

Onset

In patients with hypertension, maximum reduction in BP usually occurs 2–4 hours after administration.

Food

Food does not affect the extent of absorption; however, absorption may be delayed.

Distribution

Extent

Not known whether prazosin crosses the placenta; distributed into milk in small amounts. Crosses the blood-brain barrier.

Plasma Protein Binding

Approximately 97%.

Elimination

Metabolism

Extensively metabolized, principally in the liver by demethylation and conjugation.

Elimination Route

Excreted principally in feces via biliary excretion and to a lesser extent in urine (6–10%).

Half-life

2–4 hours.

Stability

Storage

Oral

Capsules

20-25°C; protect from moisture and light.

Actions

  • Reduces peripheral vascular resistance and BP as a result of vasodilating effects; produces both arterial and venous dilation.

  • Effects appear to result from selective, competitive inhibition of α1-adrenergic receptors.

  • Generally causes no change in heart rate, cardiac output, renal blood flow, and GFR.

  • Binds to α-adrenergic receptors on the prostate capsule, prostate adenoma, and bladder trigone, resulting in decreased urinary outflow resistance in men.

  • May improve to limited extent the serum lipid profile (e.g., small increases in HDL and HDL/total cholesterol ratio; small decreases in LDL, total cholesterol, and triglyceride concentrations).

  • Precise mechanism of action in PTSD not fully elucidated; however, norepinephrine and α1-adrenergic receptors play an important role in the pathophysiology of PTSD-associated nightmares, arousal, selective attention and vigilance. Prazosin is believed to help correct the effects of α1-adrenergic receptor hyperstimulation in PTSD and has also been shown to normalize the sleep cycle.

Advice to Patients

  • Possible dizziness, lightheadedness or fainting, especially at initiation of therapy; importance of avoiding driving or other hazardous tasks where injury could occur for 24 hours after the first dose or when dosage is increased.

  • Importance of advising patient that alcohol use, hot weather, exercise, and standing for long periods of time may precipitate or exacerbate symptoms of dizziness, lightheadedness, or fainting, and to use caution during these situations.

  • Importance of sitting or lying down when symptoms of lowered BP occur, and of rising carefully from a sitting or lying position.

  • Possible drowsiness or somnolence; use caution when operating machinery or driving a motor vehicle until effects on individual are known.

  • Importance of advising patients being considered for cataract surgery that they should inform their ophthalmologist of current or prior α1-blocker therapy, including prazosin.

  • Importance of advising patients receiving prazosin for PTSD that the drug may help reduce nightmares and improve sleep and other symptoms; however, it does not cure PTSD and their nightmares, anxiety, and other PTSD-related symptoms may return if therapy is stopped.

  • Importance of women informing clinicians if they are or plan to become pregnant or plan to breast-feed.

  • Importance of informing clinicians of existing or contemplated concomitant therapy, including prescription and OTC drugs, as well as concomitant illnesses.

  • Importance of informing patients of other important precautionary information. (See Cautions.)

Preparations

Excipients in commercially available drug preparations may have clinically important effects in some individuals; consult specific product labeling for details.

Please refer to the ASHP Drug Shortages Resource Center for information on shortages of one or more of these preparations.

* available from one or more manufacturer, distributor, and/or repackager by generic (nonproprietary) name

Prazosin Hydrochloride

Routes

Dosage Forms

Strengths

Brand Names

Manufacturer

Oral

Capsules

1 mg (of prazosin)*

Minipress

Pfizer

Prazosin Hydrochloride Capsules

2 mg (of prazosin)*

Minipress

Pfizer

Prazosin Hydrochloride Capsules

5 mg (of prazosin)*

Minipress

Pfizer

Prazosin Hydrochloride Capsules

AHFS DI Essentials™. © Copyright 2021, Selected Revisions April 8, 2019. American Society of Health-System Pharmacists, Inc., 4500 East-West Highway, Suite 900, Bethesda, Maryland 20814.

† Use is not currently included in the labeling approved by the US Food and Drug Administration.

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