Tramadol hydrocholoride is a medication used for pain control in humans due to its opioid effects on the central nervous system. At the current time in the veterinary world, it is used primarily in dogs and cats although not yet labeled for veterinary use. At the 2011 American Association of Equine Practitioners convention, held Nov. 18-22 in San Antonio, Texas, Heather Knych, DVM, PhD, of the University of California, Davis, discussed a trial in which she and colleagues administered this pharmaceutical drug to horses to evaluate its pharmacokinetic (the action of drugs in the body over a period of time) and behavioral effects.

"We are trying to find a drug that achieves analgesia with minimal side effects," Knych explained.

Generally, researchers determine what concentration of a drug in the plasma is therapeutic, or helpful for the patient. Previous studies have failed to achieve in horses what are considered therapeutic tramadol plasma concentrations for humans. In this study the research team evaluated three different dosage strengths administered to nine mature horses averaging 10 years of age. The horses received no medications for the two weeks prior to the study. Tramadol tablets, dissolved in water, were given by nasogastric tube. In a follow-up study a month later, the horses received three different dosage strengths of a compounded tramadol formulation intravenously.

Knych described the measurements used to evaluate the degree of the medication’s behavioral effects: a) chin-to-ground distance; and b) step counts, which would likely be increased as a result of opiate-induced excitation. The investigators placed measuring devices on the limbs to count the number of steps, and they used a Holter monitor to evaluate each horse’s heart rate and rhythm for four to six hours following treatment