Phenylbutazone

Oxyphenbutazone, the major metabolite of phenylbutazone, differs only in the para location of one of its phenyl groups, where a hydrogen atom is replaced by a hydroxyl group (making it 4-butyl-1-(4-hydroxyphenyl)-2-phenyl-3,5-pyrazolidinedione).

Despite its name, phenylbutazone is chemically unrelated to the class of chemicals known as benzones (common examples include oxybenzone, dioxybenzone, avobenzone, and sulisobenzone), which are used as active ingredients in sunscreen formulations for protection against UVB rays.

Phenylbutazone is used as a non-steroidal anti-inflammatory drug (NSAID) for the treatment of chronic pain, including the symptoms of arthritis. Its use is limited by such severe side effects as suppression of white blood cell production and aplastic anaemia.

The International Agency for Research on Cancer (IARC) places it in Group 3; ie. "not classifiable as to its carcinogenicity to humans."

Uses of phenylbutazone in horses

Phenylbutazone is an NSAID commonly used in horses for the following purposes:

  • Analgesia: Pain relief from infections and musculoskeletal disorders including sprains, overuse injuries, tendonitis, arthralgias, arthritis, and laminitis. Like other NSAIDs, phenylbutazone acts directly on musculoskeletal tissue to control inflammation, thereby reducing secondary inflammatory damage, alleviating pain, and restoring range of motion. Does not cure musculoskeletal ailments or work well on colic pain.
  • Antipyresis: Reduction of fevers. The antipyretic qualities of phenylbutazone may mask other symptoms; therefore, the drug should not be administered for this purpose unless a veterinarian has concluded that the horse would not be able to eat or drink without its use or that the fever might hinder the horse's recovery.

    • Dosage and administration

    Phenylbutazone may be administered orally (via paste, powder or feed-in) or intravenously. It should not be given intramuscularly or injected in any place other than a vein, as it can cause tissue damage. Tissue damage and oedema may also occur if the drug is injected repetitively into the same vein.

    The maximum oral dose recommended by manufacturers is 2 to 4 grams per 1000 pounds of body weight (4 to 9 mg/kg) per day. Manufacturers recommend that the dose be divided equally and given every 8 hours for maximum results, although most horse owners give it every 12 to 24 hours for convenience, usually giving 1 to 2 grams in the morning and at night.

    Intravenously, the maximum daily dose recommended by manufacturers is 1 to 2 grams per 1000 pounds (2 to 4.5 mg/kg). The maximum dose is usually given when the course of treatment is initiated, with the dosage subsequently being titrated down.

    Phenylbutazone should be administered only under the advice of a veterinarian.

    Side effects and disadvantages of phenylbutazone

    Side effects of phenylbutazone are similar to that of other NSAIDs. Overdose or prolonged use can cause gastrointestinal ulcers, kidney damage, oral lesions, and internal haemorrhage, especially pronounced in young, ill, or stressed horses. Effects of gastrointestinal damage include oedema of the legs and belly secondary to leakage of blood proteins into the intestines, resulting in decreased appetite, excessive thirst, weight loss, weakness, and in advanced stages, kidney failure and death.

    Phenylbutazone should not be used in combination with blood thinners (eg. Coumadin), as it amplifies the anticoagulant effects of these drugs; with other NSAIDs (all NSAIDs are additive); or in horses with known kidney or liver problems.

    Phenylbutazone should be used cautiously in pregnant or nursing mares, as it may be toxic to the embryo and can be transferred via the umbilical cord and by milk.

    High doses of phenylbutazone may be considered a rules violation under some equestrian organisations, as the drug may remain in the bloodstream four to five days after administration.

    History of phenylbutazone in racing

    In the 1968 Kentucky Derby, Dancer's Image, the winner of the race, was disqualified after traces of phenylbutazone were discovered in a post-race urinalysis. Owned by prominent Massachusetts businessman Peter Fuller and jockeyed by Bobby Ussery, Dancer's Image remains the only horse in history to win the Kentucky Derby and then be disqualified. After many appeals Forward Pass was named the winner.

    To this day controversy surrounds the 1968 Kentucky Derby. Phenylbutazone was legal on most tracks around the country in 1968 and was legal at Churchill Downs the year before and after. Also, in the weeks prior to the race Peter Fuller had given his purse to Coretta Scott King, the widow of slain Civil Rights activist Martin Luther King Jr., an act that brought him both praise and criticism. Martin Luther King Jr. had organised a sit-in on the Kentucky Derby the year before.

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