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Phenoxybenzamine for Dogs, Cats, and Horses

By Barbara Forney, VMD

Last reviewed: 9/20/2022

Commonly prescribed for: Difficulty urinating due to spasms in the urethra; reduce blood pressure in animals with a certain type of tumor called pheochromocytoma' used in horses to decrease urethral sphincter tone

Species: Dogs, Cats, and Horses

Therapeutic Class: Antihypertensives

General Drug Information and Indications

Phenoxybenzamine is used in dogs and cats that have difficulty urinating due to spasms in the urethra. It is also used to reduce blood pressure in animals with a certain type of tumor called pheochromocytoma. This rare tumor of the adrenal gland most commonly occurs in older dogs, but may also occur in cats. Phenoxybenzamine is used in horses to decrease urethral sphincter tone, and there are some experimental reports on its use in the early stages of laminitis and for some types of diarrhea.

Like many other drugs in veterinary medicine, this drug is not FDA-approved for use in animals and is not available from a veterinary pharmaceutical manufacturer. Instead, it is compounded by a specialty pharmacy.

How to Give this Medication

Give this medication to your pet exactly as your veterinarian prescribes. If you miss giving your pet a dose of phenoxybenzamine, give the next dose as soon as you remember, or, if it is close to the next scheduled dose, return to the regular schedule. Do not double dose in order to catch up.

Give with food to reduce GI (stomach) upset.

Wash your hands after giving your pet this medication.

Side Effects

Be sure to discuss any side-effects with your veterinarian immediately.

Phenoxybenzamine can cause changes in blood pressure, most often low blood pressure. Other side effects include rapid heart beat, weakness, dizziness, nasal congestion, pinpoint pupils, inability to ejaculate, nausea, and vomiting.

Phenoxybenzamine can cause constipation and colic in horses.


Keep this and all drugs out of reach of children. Phenoxybenzamine is a prescription drug and should be used according to your veterinarian's directions. It should only be given to the animal for which it was prescribed. Do not give this medication to a person.

Phenoxybenzamine should be avoided in animals that are in shock, dehydrated, or have decreased kidney function. Your veterinarian may choose to use intravenous fluids to support your pet under these circumstances.

Phenoxybenzamine should be used with caution in animals with congestive heart failure or other heart problems.

Phenoxybenzamine should not be used in animals with glaucoma or diabetes mellitus.

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

Drug Interactions

Be sure to review with your veterinarian any medications or supplements your pet may be receiving.

The following drugs may interact with phenoxybenzamine: epinephrine, phenylephrine, and reserpine.


If you suspect your pet or another animal was overdosed accidentally or has eaten this medication inadvertently, contact your veterinarian or the A.S.P.C.A.'s Animal Poison Control Center at 888.426.4435. Always bring the prescription container with you when you take your pet for treatment.

If you or someone else has accidentally ingested this medication call the National Capital Poison Center at 800.222.1222.


Different strengths or dosage forms of phenoxybenzamine may have different storage requirements. Read the labeling or ask your pharmacist for the storage requirements of the prescription you receive.
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