Side Effects and Disadvantages
Side effects of phenylbutazone are similar to those of other NSAIDs. Overdose or prolonged use can cause GI ulcers, blood dyscrasia, kidney damage, oral lesions, and internal hemorrhage, especially pronounced in young, ill, or stressed horses. Effects of GI damage include edema 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 antiplatelet drugs (e.g., coumadin or warfarin), 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 displaces warfarin from plasma binding sites and toxic blood levels leading to haemorrhage can occur.
Periodic blood tests are recommended when using phenylbutazone, as agranulocytosis can occur.
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.
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.
High doses of phenylbutazone may be considered a rules violation under some equestrian organizations, as the drug may remain in the bloodstream four to five days after administration.
The International Agency for Research on Cancer places it in Group 3; i.e., "not classifiable as to its carcinogenicity to humans".
Use in horses is limited to those not intended for food. Metabolites of phenylbutazone can cause aplastic anaemia in humans.
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