doxazosin mesylate

Action

Blocks alpha₁-adrenergic receptors, promoting vasodilation. Also reduces urethral resistance, relieving obstruction and improving urine flow and other symptoms of benign prostatic hypertrophy (BPH).

Availability

Tablets: 1 mg, 2 mg, 4 mg, 8 mg

Tablets (extended-release): 4 mg, 8 mg

Indications and dosages

➣ Hypertension

Adults: 1 mg P.O. once daily. May increase dosage gradually q 2 weeks, up to 2 to 16 mg daily, as needed.

➣ BPH

Adults: 1 mg P.O. once daily. May increase dosage gradually, up to 8 mg daily, as needed. Or, initially 4 mg (extended-release) P.O. daily. May increase dosage to 8 mg daily, as needed, at 3- to 4-week intervals.

Off-label uses

• Pheochromocytoma

• Syndrome X

Contraindications

• Hypersensitivity to drug, its components, or quinazoline derivatives

Precautions

Use cautiously in:

• renal or mild or moderate hepatic impairment, coronary insufficiency, or preexisting severe GI narrowing

• severe hepatic impairment (extended-release form not recommended)

• intraoperative floppy iris syndrome

• concurrent use of strong CYP3A4 inhibitor (such as atazanavir, clarithromycin, indinavir, itraconazole, ketoconazole, nefazodone, nelfinavir, ritonavir, saquinavir, telithromycin, or voriconazole), phosphodiesterase-5 (PDE-5) inhibitors

• elderly patients

• pregnant or breastfeeding patients (extended-release form not recommended in breastfeeding patients)

• children (safety not established).

Administration

• Give initial immediate-release dose at bedtime to minimize orthostatic hypotension and syncope.

• Give initial extended-release dose at breakfast.

• Be aware that extended-release tablets aren't indicated for hypertension.

• Be aware that prostate carcinoma should be ruled out before giving drug for BPH.

• Know that incidence of orthostatic hypotension increases greatly when daily dosage exceeds 4 mg and that it usually occurs within 6 hours of administration.

See If new or worsening signs or symptoms of angina pectoris occur, discontinue drug.

Adverse reactions

CNS: dizziness, vertigo, headache, depression, drowsiness, fatigue, nervousness, weakness, asthenia

CV: orthostatic hypotension, chest pain, palpitations, tachycardia, arrhythmias

EENT: abnormal or blurred vision, conjunctivitis, epistaxis, rhinitis, pharyngitis

GI: nausea, vomiting, diarrhea, constipation, abdominal discomfort, flatulence, dry mouth

GU: decreased libido, sexual dysfunction

Respiratory: dyspnea

Musculoskeletal: joint pain, arthritis, gout, myalgia

Skin: flushing, rash, pruritus

Other: edema

Interactions

Drug-drug. Clonidine, nitrates, other antihypertensives: decreased antihypertensive effect

Drugs that reduce GI motility leading to markedly prolonged GI retention times (such as anticholinergics): increased systemic exposure to doxazosin

PDE-5 inhibitors: increased risk of symptomatic hypotension

Drug-diagnostic tests. Neutrophils, white blood cells: decreased counts

Drug-food. Any food: increased drug plasma C_max (extended-release form)

Patient monitoring

• Monitor blood pressure with patient lying down and standing up every 2 to 6 hours after initial dose or after a dosage increase (when orthostatic hypotension is most likely to occur).

Patient teaching

• Tell patient to swallow extended-release tablets whole and not to chew, divide, cut, or crush them.

• Caution patient not to drive or perform other activities requiring alertness for 12 to 24 hours after first dose.

• Tell patient to move slowly when sitting up or standing, to avoid dizziness or light-headedness from sudden blood pressure decrease.

• Advise patient to report episodes of dizziness or palpitations.

• As appropriate, review all other significant and life-threatening adverse reactions and interactions, especially those related to the drugs and tests mentioned above.