Prazosin


Generic Medicine Info
Indications and Dosage
Oral
Raynaud's syndrome
Adult: Initially, 0.5 mg bid for 3-7 days; adjust gradually according to patient’s response. Usual maintenance dose: 1-2 mg bid.
Elderly: Initiate at the lower end of the dosing range.

Oral
Congestive heart failure
Adult: Initially, 0.5 mg 2-4 times daily; increase gradually to 4 mg, according to response. Usual maintenance dose: 4-20 mg daily in divided doses.
Elderly: Initiate at the lower end of the dosing range.

Oral
Hypertension
Adult: Initially, 0.5 mg bid or tid for 3-7 days to be taken in the evening, then increase to 1 mg bid or tid for the next 3-7 days, if tolerated. Then gradually increase up to Max 20 mg daily in divided doses according to response. Alternatively, initial dose of 1 mg bid or tid, then titrate dose up to 20 mg daily in 2-3 divided doses according to response.
Elderly: Initiate at the lower end of the dosing range.

Oral
Benign prostatic hyperplasia
Adult: As an adjunct: Initially, 0.5 mg bid for 3-7 days; adjust according to patient’s response. Usual maintenance dose: 2 mg bid.
Elderly: Initiate at the lower end of the dosing range.
Special Patient Group
Hypertension
Patients taking other antihypertensives but with inadequate control: Initially, 0.5 mg in the evening, then continue with 0.5 mg bid or tid; increase gradually thereafter according to patient’s response.
Renal Impairment
Moderate to severe: Initially, 0.5 mg daily, increase cautiously.
Hepatic Impairment
Initially, 0.5 mg daily, increase cautiously.
Administration
May be taken with or without food. Starting dose is best taken w/ dinner, at least 2-3 hr before retiring. Maintenance doses may be taken w/ or w/o meals.
Special Precautions
Patient with history of micturition syncope in the treatment of benign prostatic hyperplasia (BPH); CHF due to mechanical obstruction (e.g. aortic valve stenosis, mitral valve stenosis, pulmonary embolism, restrictive pericardial disease); ischaemic heart disease. Cataract surgery patient. Hepatic and moderate to severe renal impairment. Elderly. Pregnancy and lactation.
Adverse Reactions
Significant: Syncope (with sudden loss of consciousness), orthostatic hypotension, priapism, prolonged erections, angina, CNS depression, intraoperative floppy iris syndrome.
Cardiac disorders: Palpitations.
Ear and labyrinth disorders: Vertigo.
Eye disorders: Blurred vision.
Gastrointestinal disorders: Diarrhoea, nausea, vomiting, dry mouth, constipation.
General disorders and administration site conditions: Oedema, lack of energy, weakness.
Nervous system disorders: Headache, dizziness, drowsiness, faintness.
Psychiatric disorders: Depression, nervousness.
Respiratory, thoracic and mediastinal disorders: Dyspnoea, nasal congestion.
Skin and subcutaneous tissue disorders: Rash.
Patient Counseling Information
This drug may cause dizziness or weakness, if affected, do not drive or operate machinery.
Monitoring Parameters
Rule out prostate cancer before initiation of therapy. Monitor blood pressure and cardiac status.
Overdosage
Symptom: Hypotension. Management: Supportive treatment. Keep the patient in a supine position to restore blood pressure and normalise the heart rate. If inadequate, treat shock with volume expanders; vasopressors (including angiotensin) should be given if needed. Monitor renal function.
Drug Interactions
May cause symptomatic hypotension with phosphodiesterase type-5 (PDE-5) inhibitors (e.g. sildenafil, tadalafil, vardenafil). Additive hypotensive effect with diuretics or other antihypertensive agents.
Lab Interference
May give false-positive results in screening tests for phaeochromocytoma urinary vanillylmandelic acid (VMA) and methoxyhydroxyphenyl glycol (MHPG) metabolites of norepinephrine.
Action
Description: Prazosin is an α-blocker that competitively inhibits postsynaptic α1-adrenoreceptors, resulting in peripheral dilatation of arteries and veins, and  decrease in total peripheral resistance and blood pressure.
Onset: Within 2 hours (antihypertensive).
Duration: 10-24 hours.
Pharmacokinetics:
Absorption: Readily absorbed from the gastrointestinal tract. Bioavailability: 43-82%. Time to peak plasma concentration: 1-3 hours.
Distribution: Crosses the placenta, enters breast milk. Volume of distribution: 0.5 L/kg. Plasma protein binding: 97%.
Metabolism: Extensively metabolised in the liver via demethylation and conjugation.
Excretion: Mainly via faeces (5-11% as unchanged drug); urine (<10%). Elimination half-life: 2-3 hours.
Chemical Structure

Chemical Structure Image
Prazosin

Source: National Center for Biotechnology Information. PubChem Compound Summary for CID 4893, Prazosin. https://pubchem.ncbi.nlm.nih.gov/compound/Prazosin. Accessed Apr. 26, 2022.

Storage
Store below 30°C. Protect from light and moisture.
MIMS Class
Other Antihypertensives
ATC Classification
C02CA01 - prazosin ; Belongs to the class of alpha-adrenoreceptor antagonists, peripherally-acting antiadrenergic agents. Used in the treatment of hypertension.
References
Anon. Prazosin. Lexicomp Online. Hudson, Ohio. Wolters Kluwer Clinical Drug Information, Inc. https://online.lexi.com. Accessed 04/02/2022.

Apotex NZ Ltd. Apo-Prazosin 1 mg, 2 mg and 5 mg Tablets 31 January 2018. Medsafe. http://www.medsafe.govt.nz. Accessed 12/01/2022.

Buckingham R (ed). Prazosin Hydrochloride. Martindale: The Complete Drug Reference [online]. London. Pharmaceutical Press. https://www.medicinescomplete.com. Accessed 12/01/2022.

Hypovase 1 mg Tablets (Pfizer Limited). MHRA. https://products.mhra.gov.uk. Accessed 12/01/2022.

Joint Formulary Committee. Prazosin. British National Formulary [online]. London. BMJ Group and Pharmaceutical Press. https://www.medicinescomplete.com. Accessed 12/01/2022.

Minipress (Pharmaniaga Manufacturing Berhad). National Pharmaceutical Regulatory Agency - Ministry of Health Malaysia. https://www.npra.gov.my. Accessed 12/01/2022.

Minipress Capsule (Pfizer Laboratories Div Pfizer Inc). DailyMed. Source: U.S. National Library of Medicine. https://dailymed.nlm.nih.gov/dailymed. Accessed 12/01/2022.

Disclaimer: This information is independently developed by MIMS based on Prazosin from various references and is provided for your reference only. Therapeutic uses, prescribing information and product availability may vary between countries. Please refer to MIMS Product Monographs for specific and locally approved prescribing information. Although great effort has been made to ensure content accuracy, MIMS shall not be held responsible or liable for any claims or damages arising from the use or misuse of the information contained herein, its contents or omissions, or otherwise. Copyright © 2022 MIMS. All rights reserved. Powered by MIMS.com
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