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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 bethanacol.
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 pre-operatively 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 intra-ocular 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.
Dr.
Barbara Forney is a veterinary practitioner in Chester County, Pennsylvania.
She has a master's degree in animal science from the University of Delaware
and graduated from the University of Pennsylvania School of Veterinary Medicine
in 1982.
She began to develop her interest in client education and medical writing 1997. Recent publications include portions of The Pill Book Guide to Medication for Your Dog and Cat, and most recently Understanding Equine Medications published by the Bloodhorse.
Dr. Forney is an FEI veterinarian and an active member of the AAEP, AVMA, and AMWA.
You can purchase books by Dr. Forney at www.exclusivelyequine.com
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The information contained on this site
is general in nature and is intended for use as an informational aid. It does
not cover all possible uses, actions, precautions, side effects, or interactions
of the products shown, nor is the information intended as medical advice or
diagnosis for individual health problems or for making an evaluation as to the
risks and benefits of using a particular product. You should consult your doctor
about diagnosis and treatment of any health problems. Information and statements
have not been evaluated by the Food and Drug Administration ("FDA"),
nor has the FDA approved the products to diagnose, cure or prevent disease.
Wedgewood compounded veterinary medicines are not intended for use in food and food-producing animals.
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