Phenoxybenzamine Hydrochloride

General

• Adjust dosage carefully according to individual requirements and response.a b

• Minimize adverse effects by starting with small doses and gradually increasing the dosage until the desired effect is obtained or adverse effects occur.a b

• If concomitant propranolol therapy is required, initiate phenoxybenzamine prior to use of propranolol and continue during propranolol therapy in order to prevent severe hypertension due to unopposed α-adrenergic stimulation.a

Administration

Oral Administration

Administer orally in divided doses.a If GI irritation occurs, administer with milk.a

Dosage

Available as phenoxybenzamine hydrochloride; dosage expressed in terms of the salt.a

Pediatric Patients

Initially, 0.2 mg/kg or 6 mg/m2 once daily; do not exceed 10 mg. a

Increase dosage gradually until an adequate response is achieved (e.g., BP is controlled). a

Usual maintenance dosage: 0.4–1.2 mg/kg or 12–36 mg/m2 daily.a

Adults

Initially, 10 mg twice daily.a b

Increase dosage gradually every other day until an adequate response is achieved (e.g., BP is controlled).a b

Usual maintenance dosage: 20–40 mg 2 or 3 times daily; higher dosages may be required.a

Initially, 10 mg twice daily. a

Increase dosage gradually every other day until an adequate response is achieved.a

Usual maintenance dosage: 20–40 mg 2 or 3 times daily; higher dosages may be required.a

Prescribing Limits

Pediatric Patients

Initially, maximum 10 mg daily.a

Special Populations

No special population dosage recommendations at this time.b

Contraindications

• Conditions where a decrease in BP is undesirable.a b

• Known hypersensitivity to phenoxybenzamine or any ingredient in the formulation.b

Warnings/Precautions

Warnings

Phenoxybenzamine’s α-adrenergic blocking effect leaves β-receptors unopposed; concomitant use with drugs that stimulate α- and β-adrenergic receptors (i.e., epinephrine) may cause an exaggerated hypotensive response and tachycardia.b (See Specific Drugs under Interactions.)

Phenoxybenzamine-induced tachycardia may precipitate CHF and angina in patients with compensated CHF or CAD.a

General Precautions

May aggravate symptoms of respiratory infections.b

Use with caution in patients with marked cerebral or coronary arteriosclerosis.b

Specific Populations

Category C.b

Not known whether phenoxybenzamine is distributed into milk.a b Caution if used in nursing women.a b

Safety and efficacy not established.b

Use with caution in patients with renal damage.b

Common Adverse Effects

Nasal congestion,a b miosis,a b postural hypotension,a dizziness,a tachycardia.a b

Absorption

Bioavailability

Variably absorbed from the GI tract;a 20–30% of an oral dose is absorbed.b

Onset

Following oral administration, onset of action is gradual over a period of several hours. a

Duration

α-Adrenergic blockade persists for 3–4 days following oral administration of a single dose; after administration of fixed daily doses, α-adrenergic blocking effects are cumulative for about 7 days.a

Distribution

Extent

Highly lipid soluble; may accumulate in fat following administration of large doses.a

Not known whether phenoxybenzamine crosses the placenta or is distributed into milk.a b

Elimination

Metabolism

Dealkylated to form N-phenoxyisopropyl-benzylamine.a

Elimination Route

Excreted in urine and bile.a

Half-life

Approximately 24 hours.a b

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

• Importance of taking only as prescribed; do not increase dosage or duration of therapy unless otherwise instructed by a clinician.a

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

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

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.