Pharmacotherapeutic group: Anti-inflammatory and antirheumatic products, non-steroids
Phenylbutazone is a pyrazolone non-steroidal anti-inflammatory drug (NSAID) with analgesic, anti-inflammatory and anti-pyretic activity. These pharmacodynamic effects are achieved by the inhibition of prostaglandin synthetase (cyclo-oxygenase).
The plasma elimination half life of phenylbutazone in the horse varies from 3.5 - 8 hours. Normally peak plasma levels are achieved approximately 2-3 hours after administration. Oral bioavailability is high but concurrent feeding of hay can delay the time to peak concentration, decreases the peak plasma concentrations and so delay the onset of a clinical effect.
Phenylbutazone binds heavily to plasma albumin.
Phenylbutazone is metabolised in the liver to oxyphenbutazone, which also has similar pharmacological activity. Further metabolism takes place to gamma- hydroxyphenylbutazone. Excretion is mainly via the urine.