Phenoxybenzamine for Veterinary Use

Therapeutic Class: Alpha-adrenergic blocker (Smooth muscle relaxant)

Species: Dogs, cats and horses

May Be Prescribed For: Reduction of urinary sphincter tone, and treatment of hypertension due to pheochromocytoma.

Phenoxybenzamine blocks the alpha adrenergic response to circulating epinephrine and norepinephrine. It has no effect on beta adrenergic receptors or on the parasympathetic nervous system. Phenoxybenzamine increases blood flow to the skin but does not affect cerebral or musculoskeletal blood flow. It has been shown to decrease both standing and supine blood pressure in humans.

Phenoxybenzamine is used in dogs and cats to reduce internal urethral sphincter tone. Although it is somewhat more effective in dogs than in cats, it is used in both species after a mechanical obstruction of the urethra has been relieved. It may also be used for urethral spasm secondary to bacterial urethritis. Phenoxybenzamine may be used concurrently with bethanechol .

Phenoxybenzamine is used to reduce blood pressure associated with pheochromocytoma. This rare neuroectodermal tumor of the chromaffin cells of the adrenal medulla most commonly occurs in older dogs but may also occur in cats. Approximately 50% of cases have episodic hypertension due to catecholamine secretion. These tumors are challenging to manage and treat due to the severity of the hypertension and cardiac arrhythmias. Phenoxybenzamine is used preoperatively in an attempt to regain control of blood pressure prior to surgery.

Phenoxybenzamine may be used to decrease urethral sphincter tone, in horses with an atonic bladder. It may be used with bethanacol. There are earlier reports of its use in the early stages of laminitis and for diarrhea. Phenoxybenzamine is an ARCI class 3 drug.

• Hypotension or hypertension, tachycardia, weakness dizziness, GI disturbances (nausea).

• Ocular side-effects include miosis and increased intraocular pressure.

• Phenoxybenzamine causes vasodilation and should be avoided in animals that are significantly dehydrated or azotemic. Hydration may be supported with intravenous fluids.

• Phenoxybenzamine should be used with caution in animals with kidney damage, congestive heart failure or other cardiac problems, and those with coronary or cerebral arteriosclerosis. It should not be used in animals with glaucoma or diabetes mellitus.

• Phenoxybenzamine should not be used in horses with signs of colic.

• Phenoxybenzamine will antagonize alpha adrenergic sympathomimetic drugs.

• The effects of phenoxybenzamine will be increased when used with drugs such as epinephrine, that have both alpha and beta adrenergic properties.

• Overdose with phenoxybenzamine may produce hypotension, tachycardia, GI distress, lethargy, and shock.

• If the overdose is treated early, gastric-emptying may be appropriate.