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Phenylbutazone is a nonsteroidal anti-inflammatory drug (NSAID) and cylo-oxygenase inhibitor. It is a potent pain reliever, antipyretic, and anti-inflammatory. In the horse, it is commonly used for lameness, resulting from from soft tissue injury, muscle soreness, bone and joint problems, and laminitis. NSAIDs work by inhibiting the body's production of prostaglandins, thromboxane, and other inflammatory mediators. Some of these actions may be dose dependent. Phenylbutazone may be given intravenously or orally; pain relief and fever reduction usually starts within one to two hours.
Phenylbutazone is occasionally used in dogs for the longer-term management of chronic pain particularly due to osteoarthritis. About 20% of adult dogs are affected with osteoarthritis, which makes the management of musculoskeletal pain a major component of companion animal practice. There is a very narrow margin of safety for all NSAIDs in the dog and there are other NSAIDs that are more commonly used (etodolac, and carprofen). GI protectant drugs such as misprostal, cimetidine, omeprazole, ranitidine, or sucralfate are frequently included as a part of treatment with any NSAID. Dogs receiving chronic phenylbutazone therapy should be followed with regular blood work and renal monitoring.
Phenylbutazone is an inexpensive generally well-tolerated drug. It is frequently the first choice for pain control of many musculoskeletal problems although other NSAIDs, such as flunixin, are more commonly used for gastro-intestinal pain or colic. Recent research into NSAID toxicity and equine gastric ulcer disease may have given phenylbutazone a bad reputation for safety. However, when used at the appropriate dose and according to directions, phenylbutazone is generally a safe and effective drug. Additional care should be shown with special populations such as foals, ponies, older horses, and debilitated or dehydrated horses. These populations are more likely to have adverse side effects.
• Dogs: GI ulceration, bone marrow depression, rashes, malaise, blood dyscrasias, diminished renal blood flow.
• Horses: The most common side effects include ulceration of the mouth and GI tract. Less common side effects include renal damage, bleeding disorders, and protein loss.
• Injection site reactions can occur if blood leaks back at the injection site. Injectable phenylbutazone is very irritating to tissue if any leaks out of the vein.
• Do not inject in the muscle, under the skin, or intra-arterially.
• NSAIDS should be avoided or very carefully monitored in animals with liver disease, kidney disease, or GI problems. Therapy should be stopped at the first sign of any adverse reaction (anorexia, oral ulcers, depression, decreased plasma protein, increased creatinine, anemia, leukopenia).
• Work in rodents indicates that phenylbutazone may be harmful to the embryo. It can cross the placenta and is found in milk. Phenylbutazone should be avoided or used with caution in pregnant or nursing animals.
• Pony breeds may be more susceptible to side effects from NSAIDs than horses. Older horses especially those with decreased kidney or liver function may also be more at risk for side effects. When NSAIDs are used in these populations, they should be used with caution and at the lowest effective dose.
• Phenylbutazone may be used in foals, but it should be used with particular caution. Premature foals, septicemic foals, foals with questionable kidney or liver function and foals with diarrhea require careful monitoring. Drugs to protect the GI tract such as omeprazole, cimetidine, and sucralfate are frequently used with phenylbutazone.
• Some veterinarians and many horse owners, particularly those involved in showing, may use more than one NSAID in combination, for example, flunixin and phenylbutazone given together. Although there is little experimental evidence to support this practice, the theory is that different NSAIDs may act differently on different body systems. Particular care needs to be taken in this situation to avoid additive toxicity.
• Avoid combining with other anti-inflammatory drugs that tend to cause GI ulcers, such as corticosteroids and other NSAIDs. Avoid combining with anticoagulant drugs particularly coumarin derivatives. Avoid combining with other hepatotoxic drugs.
• Phenylbutazone may affect blood levels and duration of action of phentoin, valproic acid, sulfonamides, sulfonylurea antidiabetic agents, barbiturates, promethazine, rifampin, chlorpheniramine, diphenhydramine, penicillin G.
Overdoses of phenylbutazone can cause renal failure, liver injury, bone marrow suppression, and gastric ulceration/perforation. Early signs of toxicity include loss of appetite, and depression.
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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