Phenoxybenzamine

Phenoxybenzamine In Dogs & Cats: Uses, Dosage and Side Effects

Overview

• An alpha-adrenergic antagonist that irreversibly blocks pre-synaptic and post-synaptic receptors, producing a chemical sympathectomy (interruption of the sympathetic nervous pathway).

Uses of Phenoxybenzamine

• Reflex dyssynergia or functional urethral obstruction due to urethral spasm.
• Treatment of severe hypertension in animals with phaeochromocytoma before surgery to reduce mortality.
• If concurrent beta-blockers are also used (for severe tachycardia/arrhythmias), only start these once alpha blockade is in place (to avoid a hypertensive crisis).
• Use with extreme caution in animals with pre-existing cardiovascular disease.

Dose of Phenoxybenzamine in Dogs and Cats

Dogs:

• Reflex dyssynergia: 0.25–1 mg/kg p.o. q8–24h for a minimum of 5 days.
• Hypertension associated with phaeochromocytoma: 0.25–0.5 mg/kg p.o. q12h for 10–14 days before surgery, titrating up to an effective dose (maximum of 2.5 mg/kg q12h) as required or administering long-term for medical management if adrenalectomy
  not possible.

Cats:

• 0.5–1 mg/kg p.o. q12h for 5 days before evaluating efficacy.

Side Effects of Phenoxybenzamine in Dogs and Cats

• Hypotension, miosis, tachycardia and nasal congestion.

Contraindications of Phenoxybenzamine in Dogs and Cats

• No information is available.

Some Notes:

• There is an increased risk of a first-dose hypotensive effect if administered with beta-blockers or diuretics.
• Phenoxybenzamine will antagonize the effects of alpha-adrenergic sympathomimetic agents (e.g. phenylephrine).