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Phenylbutazone for Dogs and Horses

By Barbara Forney, VMD

Commonly prescribed for: Pain relief particularly for musculoskeletal pain, osteoarthritis, anti-inflammatory, antipyretic

Species: Dogs and Horses

Therapeutic Class: Nonsteroidal Anti-inflammatory Drug (NSAID)


Basic Information

Phenylbutazone is a nonsteroidal anti-inflammatory drug (NSAID) and cyclooxygenase inhibitor. It is a potent pain reliever, antipyretic, and anti-inflammatory. In the horse, it is used commonly for lameness, resulting 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.

Dogs

Phenylbutazone is used occasionally in dogs for the longe-term management of chronic pain particularly due to osteoarthritis. About 20% of adult dogs are affected with osteoarthritis, which makes managing 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 used more commonly (etodolac and carprofen).

GI-protectant drugs such as Misoprostol, cimetidine, omeprazole, ranitidine, or sucralfate frequently are included as a part of treatment with any NSAID. Dogs receiving chronic phenylbutazone therapy should be followed with regular blood work and renal monitoring.

Horses

Phenylbutazone is an inexpensive, generally well-tolerated drug. It frequently is the first choice for pain control of many musculoskeletal problems although other NSAIDs, such as flunixin, are used more commonly for gastrointestinal 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 generally is 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.

Phenylbutazone 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.

Precautions

  • 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 (anorexiaoral 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 also may 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 used frequently 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.

Drug Interactions

  • 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 phenytoin, valproic acid, sulfonamides, sulfonylurea antidiabetic agents, barbiturates, promethazine, rifampin, chlorpheniramine, diphenhydramine, penicillin G.

Overdose

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.

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