Phenoxybenzamine

What is phenoxybenzamine?

Phenoxybenzamine (brand names: Dibenzyline®, Dibenyline®, Dibenzyran®, Fenoxene®) is an alpha-adrenergic blocker used to reduce urinary sphincter tone to relax the bladder and allow easier urination.

It is also used to treat high blood pressure related to pheochromocytoma; as an adjunctive treatment for endotoxicosis; and as a treatment for laminitis in horses.

Its use in cats, dogs, and horses to treat high blood pressure and high urinary sphincter tone off label (extra label). Many drugs are commonly prescribed for off-label use in veterinary medicine. 

“Off label” means the drug is being used in a way that has not been reviewed or approved by the U.S. Food and Drug Administration (FDA) or Health Canada. In these instances, follow your veterinarian’s directions and cautions carefully, as they may be significantly different from those on the label.

How is phenoxybenzamine given?

• Phenoxybenzamine is given by mouth in the form of a capsule. It may also be specially compounded into a liquid form.
• Measure liquid forms carefully.
• It may be given with or without food.
• If vomiting occurs when dosed on an empty stomach, give future doses with food.

This medication should take effect within 1 to 2 days, and improvements in clinical signs should follow.

What if I miss giving my pet the medication?

If you miss a dose, give it as soon as you remember, and then continue with the regular dosing schedule. However, if it is almost time for the next dose, skip the missed dose and resume the regular dosing schedule. Do not give the pet two doses at once.

Are there any potential side effects?

Common side effects include:

• Nausea, vomiting, diarrhea
• Small pupils
• Increased heart rate
• Nasal congestion/stuffy nose
• Constipation (in horses)

Serious adverse reactions may occur. If any of the following signs develop, stop giving phenoxybenzamine and contact your veterinarian immediately:

• Excessive tiredness
• Fainting, weakness, or dizziness

Other side effects may occur. If you notice anything unusual, contact your veterinarian immediately.  This moderate-acting medication should stop working in a few days, although effects can be longer in pets with liver or kidney disease.

Are there any risk factors for this medication?

DO NOT use phenoxybenzamine in:

• Pets that are allergic to it
• Pets that do not tolerate low blood pressure well
• Horses with colic

USE WITH CAUTION in:

• Pets with glaucoma
• Pets with diabetes
• Pets with kidney disease
• Pets with congestive heart failure or other heart disease
• Male breeding animals
• Pregnant or lactating animals (use only when the benefits outweigh the risks)

Are there any drug interactions I should be aware of?

Several medications should be used with caution when given with phenoxybenzamine, including:

• Anti-hypertensive agents (medications that treat high blood pressure)

Certain medications may interact with phenoxybenzamine, so be sure to tell your veterinarian about any medications (including vitamins, supplements, or herbal therapies) that your pet is taking before starting any new treatment.

Is any monitoring needed with this medication?

• Your veterinarian will let you know when and how they want to reassess your pet to ensure the medication is working and not causing side effects.
• Your veterinarian may monitor for adequate urination and monitor blood pressure.
• At home, monitor for any side effects such as those noted above. If any side effects occur, stop using the medication and contact your veterinarian.

How do I store phenoxybenzamine?

• Store the capsules at room temperature and protect from moisture and light.
• For compounded formulations, follow the storage instructions on the label.

What should I do in case of emergency?

If you suspect an overdose or an adverse reaction to the medication, call your veterinary office immediately. If they are not available, follow their directions in contacting an emergency facility. Be prepared to provide information about the product and your pet’s weight and symptoms.