Intravenous PBZ Dosing in Horses (AAEP 2011)

While phenylbutazone (PBZ), commonly known as "Bute," is one of the oldest and most commonly used non-steroidal anti-inflammatory drugs (NSAIDs) in horses, studies about optimal dosage are scarce in the scientific literature. Working to further cumulative veterinary knowledge about Bute dosing, Jonathan Foreman, DVM, MS, Dipl. ACVIM, and his colleagues at the University of Illinois, measured the pain relief that varying doses of intravenous (IV) PBZ generated. Foreman presented their results at the 2011 American Association of Equine Practitioners convention, held Nov. 18-22 in San Antonio, Texas.

The research group used an adjustable heart-bar shoe model to produce lameness in study horses. By applying adjustable amounts of frog pressure via the turn of a screw, this model produces an on-demand, immediately reversible, NSAID-responsive lameness. Because investigators can produce and remove lameness virtually instantaneously, each horse can act as its own control, so pain measurements are less affected by individual variation among horses.

In Foreman's study, investigators measured pain by observing heart rate elevation and lameness score. Heart rate rose in the study horses in a dose-dependent manner with the level of pressure applied to the frog, mirroring the lameness score.

For the purposes of this study, Foreman and his colleagues determined that a "single IV dose" of PBZ for a 1,000-pound horse would be 10 mL (2g). Because veterinarians often adjust PBZ doses in the clinical setting--tapering a dose downward as lameness resolves or doubling the dose in severely painful animals--Foreman's team chose to study the following dosage groups: a negative (saline) control, half dose (1g IV/1,000 lbs body weight), a single dose, and a double dose (4g IV/1,000 lbs).

The researchers found that with the 2g dose of PBZ, heart rate declined, indicating a reduction of pain, within three hours. Single and double doses both decreased heart rate and lameness scores, but there was no significant pain relief difference between the two doses, indicating that a single dose was just as effective in controlling pain as a double dose. The 1g dose produced better pain relief than the negative control, but the effects on heart rate lasted only 80 minutes.

Foreman concluded that when dosing PBZ in the horse, "More is not better. Less is less effective." It is advisable for horse owners to consult a veterinarian to determine an appropriate amount of PBZ for a given situation prior to administering the medication.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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