Side Effects of Sedative Reversal Agents in Horses (AAEP 2010)

The idea of being able to "unsedate" or "reverse" a sedated horse after minor procedures is a great one, but veterinarians must select their cases carefully and be vigilant about using these drugs properly.

"The α-2 adrenergic agonists xylazine, detomidine, and romifidine are all commonly used sedatives in equine practice," relayed David B. Scofield, DVM, of Colorado State University's College of Veterinary Medicine & Biomedical Sciences, during a presentation at the 2010 American Association of Equine Practitioners Convention, held Dec. 4-8 in Baltimore, Md. "Their effects can be reversed by such drugs as tolazoline and yohimbine."

Tolazoline is FDA-approved for horses and has been deemed a safe pharmacologic product: "Yohimbine is approved for use in dogs, so there is no label claim for the horse. Tolazoline is specifically labeled for the horse, therefore deemed safe for use in the horse."

To document the side effects associated with tolazoline or yohimbine in horses, Scofield and colleagues collected data from case reports on the AAEP e-mail distribution list and Equine Clinician's distribution list in early 2010.

"Thousands of doses of tolazoline and yohimbine are used successfully each year without incident," he said. "This survey found only 18 adverse reactions that occurred without a pattern or predisposing factors."

Of these 18 reports from veterinarians, seven horses were treated with tolazoline and 11 with yohimbine. Four and nine deaths occurred respectively following administration of these drugs.

Veterinarians are well aware that no drug is risk-free; however, Scofield et al. warned that adverse reactions to α-2 antagonist agents like tolazoline and yohimbine can occur unpredictably and without a predisposing factor.

In horses that exhibit a reaction to an α-2 antagonist, the study authors suggested providing cardiovascular support via intravenous fluid therapy and administering a vasoactive drug such as phenylephrine.

"As outlined in our paper included in the AAEP conference proceedings, veterinarians are encouraged to follow the dosage time frame and routes of administration to limit the potential for an adverse reaction and to carefully weigh the benefits of the use of these drugs for nonemergency situations," Scofield concluded.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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