Drug information of Phenylbutazone


Drug group:

A drug that has anti-inflammatory, antipyretic, and analgesic activities. It is especially effective in the treatment of ankylosing spondylitis. It also is useful in rheumatoid arthritis and Reiter's syndrome (investigational indication). Although phenylbutazone is effective in gouty arthritis, risk/benefit considerations indicate that this drug should not be employed for this disease

Mechanism of effect

Phenylbutazone binds to and inactivates prostaglandin H synthase and prostacyclin synthase through peroxide (H2O2) mediated deactivation. The reduced production of prostaglandin leads to reduced inflammation of the surrounding tissues


Phenylbutazone is a synthetic, pyrazolone derivative. It is a nonhormonal anti-inflammatory, antipyretic compound useful in the management of inflammatory conditions. The apparent analgesic effect is probably related mainly to the compound's anti-inflammatory properties and arise from its ability to reduce production of prostaglandin H and prostacyclin. Prostaglandins act on a variety of cells such as vascular smooth muscle cells causing constriction or dilation, on platelets causing aggregation or disaggregation and on spinal neurons causing pain. Prostacylcin causes vascular constriction platelet disaggregation


Phenylbutazone is rapidly and completely absorbed from the GI tract or the rectum. More than 98% of the drug is bound to plasma proteins. In humans, phenylbutazone undergoes extensive biotransformation. Hydroxylation of the phenyl rings or the butyl side chain and glucuronidation are the most significant primary reactions. Active metabolites with long plasma half-lives are formed. Half-lives of phenylbutazone in several species are shown in Table 4-3. This drug, unlike salicylate, is slowly eliminated from ruminants. The reason for this is not known (Davis, 1983). As previously mentioned, dosing intervals must be adjusted relative to the half-life of the drug: the longer the half-life, the greater the dosing interval. In turn, the time taken to reach therapeutic blood concentration is lengthened.

Drug indications

For the treatment of backache and ankylosing spondylitis

Drug contraindications

serious hepatic, renal or cardiac pathology, or those with a history of blood dyscrasia

Side effects

GI ulceration, bone marrow depression, rashes, malaise, blood dyscrasias, diminished renal blood flow
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.

  • 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


  • 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

Pregnancy level


Phenylbutazone should be avoided or used with caution in pregnant or nursing

Breast feeding warning

Phenylbutazone should be avoided or used with caution in pregnant or nursing

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