FEI (Fédération Internationale Equestre) has a drug-free policy for all equine disciplines. Recently, there was talk of modifying the drug rule to allow veterinarian-administered half-doses of flunixin meglumine or phenylbutazone (PBZ, Bute). After an exhaustive examination of the legal implications and the ethics of fairness, FEI officials abandoned this 2010 rule modification, but the discussion caused one researcher to question whether alternate dosing approaches would help ensure testing compliance.

Jonathan Foreman, DVM, MS, Dipl. ACVIM, of The University of Illinois College of Veterinary Medicine, wondered if half-doses of a non-steroidal anti-inflammatory drug (NSAID) could result in different plasma concentrations than full doses. He presented his study at the 2012 American Association of Equine Practitioners’ (AAEP) convention, held Dec. 1-5 in Anaheim, Calif.

Foreman and colleagues studied the impact of this dosing approach with Bute on nine healthy horses. They administered five consecutive once-daily half-doses of Bute either intravenously (IV) or orally, taking blood samples 12 and 24 hours post-administration. After a week off, in the third week the researchers swapped the groups’ administration method.

When the research team compared oral versus IV dosing, they noticed PBZ concentrations in plasma accumulated over time, with concentrations higher on Day 5 than Day 1. They didn’t note this accumulation with IV dosing. Foreman suggested that even at half-doses, oral Bute is not 100% bioavailable and, therefore, it tends to accumulate in the blood plasma. After discontinuing oral dosing, plasma concentrations decreased to a level that a drug test will not detect within 12 hours of dosing; horses on an IV dose tested close to a positive level but their plasma levels declined quickly after dosing stopped