Adult: As monotherapy or in combination with other antihypertensive agents: As conventional tab: Initially, 1 mg once daily in the morning or evening; may double the daily dose at intervals of 1-2 weeks according to individual response and tolerability. Usual maintenance dose: Up to 4 mg once daily. Max: 16 mg once daily. As extended-release tab: Initially, 4 mg once daily, may increase after 4 weeks up to Max of 8 mg once daily based on patient response.
Oral Benign prostatic hyperplasia
Adult: For the relief of associated symptoms and urinary outflow obstruction in hypertensive or normotensive patients: As conventional tab: Initially, 1 mg once daily in the morning or evening; may double the daily dose at intervals of 1-2 weeks according to individual response and tolerability. Usual maintenance dose: 2-4 mg daily. Max: 8 mg daily. As extended-release tab: Initially, 4 mg once daily, may increase after 4 weeks up to Max of 8 mg once daily based on patient response and tolerability. Treatment guidelines may vary among individual products or between countries (refer to specific product recommendations).
Hepatic Impairment
Severe (Child-Pugh class C): Not recommended.
Administration
May be taken with or without food.
Contraindications
History of orthostatic hypotension, benign prostatic hyperplasia (BPH) and concomitant upper urinary tract congestion, chronic urinary tract infection or bladder stones. As monotherapy in patients with overflow bladder, or anuria. Hypotension (when used for BPH).
Special Precautions
Patient with angina pectoris, recent MI within the last 6 months, high-output heart failure, right-sided heart failure due to pulmonary embolism or pericardial effusion, left ventricular heart failure with low filling pressure, pulmonary oedema due to aortic or mitral stenosis; increased gastrointestinal retention, such as chronic constipation (for extended-release tab). Patient undergoing cataract surgery. Hepatic impairment. Elderly. Pregnancy and lactation.
Adverse Reactions
Significant: Orthostatic hypotension, syncope, CNS depression, intraoperative floppy iris syndrome (particularly in cataract surgery patients); hypersensitivity reactions, including pruritus, rash, angioedema, and respiratory effects; decreased WBC and neutrophil counts. Rarely, priapism. Cardiac disorders: Tachycardia, palpitations, chest pain. Ear and labyrinth disorders: Vertigo, tinnitus. Eye disorders: Visual disturbances. Gastrointestinal disorders: Nausea, abdominal pain, dyspepsia, dry mouth. General disorders and administration site conditions: Asthenia, peripheral oedema, fatigue, malaise, influenza-like symptoms. Musculoskeletal and connective tissue disorders: Myalgia, arthralgia, back pain, muscle cramps. Nervous system disorders: Headache, dizziness, drowsiness. Psychiatric disorders: Insomnia, anxiety. Renal and urinary disorders: UTI, urinary incontinence, cystitis, polyuria. Respiratory, thoracic and mediastinal disorders: Dyspnoea, respiratory tract infection, cough, bronchitis, rhinitis, epistaxis. Vascular disorders: Hypotension, flushing.
This drug may cause dizziness, drowsiness, or weakness, if affected, do not drive or operate machinery.
Monitoring Parameters
Rule out prostate cancer before initiation of BPH therapy. Monitor blood pressure regularly and for at least 6 hours following the 1st dose and with each dose increase; CBC, LFTs and urinalysis. Assess for symptoms of hypotension regularly during treatment.
Overdosage
Symptom: Hypotension. Management: Supportive treatment. Place the patient in a supine, head down position. If inadequate, treat shock with volume expanders. May give vasopressors if needed. Monitor renal function.
Drug Interactions
May result in symptomatic hypotension with phosphodiesterase-5 (PDE-5) inhibitors (e.g. sildenafil, tadalafil, vardenafil). Strong CYP3A4 inhibitors (e.g. clarithromycin, itraconazole, ritonavir, nelfinavir, telithromycin) may increase the serum concentration of doxazosin. May potentiate the hypotensive effects of other α1-blockers and antihypertensive agents.
Lab Interference
May interfere with plasma renin activity and urinary excretion of vanillylmandelic acid.
Action
Description: Mechanism of Action: Doxazosin, a quinazoline derivative, selectively and competitively blocks the postsynaptic α1-adrenergic receptors resulting in venous and arterial vasodilation, and reduction in total peripheral resistance and blood pressure. It also competitively inhibits the postsynaptic α1-adrenergic receptors in the prostatic and bladder neck tissues, thereby decreasing the sympathetic tone-induced urethral stricture in BPH. Duration: >24 hours. Pharmacokinetics: Absorption: Well absorbed from the gastrointestinal tract. Bioavailability: Approx 65%. Time to peak plasma concentration: 2-3 hours (conventional tab); 8 ± 3.7 to 9 ± 4.7 hours (extended-release tab). Distribution: Crosses the placenta; enters breastmilk (small amounts). Plasma protein binding: Approx 98%. Metabolism: Extensively metabolised in the liver primarily by CYP3A4, and to a lesser extent by CYP2D6 and CYP2C9 isoenzymes via O-demethylation or hydroxylation. Excretion: Mainly via faeces (approx 63% mainly as metabolites, 4.8% as unchanged drug); urine (9% mainly as metabolites). Elimination half-life: Approx 22 hours (conventional tab); 15-19 hours (extended-release tab).
Chemical Structure
Doxazosin Source: National Center for Biotechnology Information. PubChem Compound Summary for CID 3157, Doxazosin. https://pubchem.ncbi.nlm.nih.gov/compound/Doxazosin. Accessed May 26, 2022.
C02CA04 - doxazosin ; Belongs to the class of alpha-adrenoreceptor antagonists, peripherally-acting antiadrenergic agents. Used in the treatment of hypertension.
References
Alfadil XL (Pfizer Pharmaceuticals LLC). MIMS Philippines. http://www.mims.com/philippines. Accessed 02/05/2022.Anon. Doxazosin Mesylate. AHFS Clinical Drug Information [online]. Bethesda, MD. American Society of Health-System Pharmacists, Inc. https://www.ahfscdi.com. Accessed 02/05/2022.Anon. Doxazosin. Lexicomp Online. Hudson, Ohio. Wolters Kluwer Clinical Drug Information, Inc. https://online.lexi.com. Accessed 02/05/2022.Buckingham R (ed). Doxazosin Mesilate. Martindale: The Complete Drug Reference [online]. London. Pharmaceutical Press. https://www.medicinescomplete.com. Accessed 02/05/2022.Cardura 1 mg Tablets (Upjohn UK Limited). MHRA. https://products.mhra.gov.uk. Accessed 02/05/2022.Cardura Tablet (Roerig). DailyMed. Source: U.S. National Library of Medicine. https://dailymed.nlm.nih.gov/dailymed. Accessed 02/05/2022.Cardura XL (Viatris Sdn. Bhd.). National Pharmaceutical Regulatory Agency - Ministry of Health Malaysia. https://www.npra.gov.my. Accessed 02/05/2022.Cardura XL Tablet, Multilayer, Extended Release (Roerig). DailyMed. Source: U.S. National Library of Medicine. https://dailymed.nlm.nih.gov/dailymed. Accessed 02/05/2022.Clinect NZ Pty Limited. Doxazosin 2 mg and 4 mg Tablets data sheet 23 August 2021. Medsafe. http://www.medsafe.govt.nz. Accessed 02/05/2022.Joint Formulary Committee. Doxazosin. British National Formulary [online]. London. BMJ Group and Pharmaceutical Press. https://www.medicinescomplete.com. Accessed 02/05/2022.Slocinx XL 4 mg Prolonged-release Tablets (Zentiva Pharma UK Limited). MHRA. https://products.mhra.gov.uk. Accessed 02/05/2022.
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