How Bute Works
Phenylbutazone is absorbed well from the stomach, but most of the dose of Bute remains in the blood and does not cross biological membranes well, Dowling explains. "The highest concentrations are in the liver, heart, kidney, lungs, and plasma, with low levels in normal tissues and joint fluid," she says. However, when tissues or joints are damaged, increased blood flow and the leaking of blood fluids from damaged blood vessels allow Bute levels to increase to therapeutic levels at these damaged sites.
"Phenylbutazone is converted by the liver to oxyphenbutazone (OPBZ), a metabolite with the same action as Bute, but that is removed slower from the body than Bute, so the therapeutic effect of Bute lasts for more than 24 hours," Dowling continues. "Less than 2% of the original dose of Bute is excreted in the urine as an unchanged drug, so liver function is very important in the elimination of this drug; the capacity of the liver to process Bute becomes overwhelmed at relatively low drug doses. Therefore, increasing doses of Bute can easily result in toxicity."
Bute is available in many intravenous and oral formulations (powder, paste, gel, tablets). "The injectable formulation must be given by careful intravenous injection, otherwise it causes severe tissue damage if given intramuscularly or subcutaneously," Dowling states. "Orally administered Bute is well absorbed, but the time it takes to reach peak blood levels is delayed by feeding the horse, as the Bute sticks to feed particles in the horse's gut."
Dosage varies somewhat depending on severity of pain or inflammation. "Routine lame horses (average horse weighing about 1,000 pounds or 450 kilograms) usually receive a dose of about 1 gram twice a day (2.2 mg/kg) initially over a five- to 10-day period," McConnico says. "However, it is not unusual for a horse to receive twice that daily dose. In fact, many veterinary textbooks and acceptable dosage recommendations are as high as 4.4 mg/kg twice a day, which is 4 grams a day."