Investigation of Metformin Contamination in Horses: Implications for Environmental Exposure and Doping Control

Metformin is a diabetes medication with minimal therapeutic efficacy in veterinary medicine. However, recently there have been several adverse analytical findings in regulatory samples collected from racehorses. Metformin is commonly prescribed to humans, is excreted unchanged in urine, and is resistant to environmental degradation, raising concerns with respect to environmental contamination. In this study, six horses were exposed to shavings contaminated with metformin-positive urine at concentrations of 10, 135, and 270 μg/mL. Blood (plasma, serum, whole blood, and red blood cells) and urine samples were collected for 168 h post-exposure for determination of metformin concentrations. All horses had detectable metformin concentrations in blood samples at least once during the sample collection period. Metformin concentrations in urine samples greatly exceeded those in blood and were detectable as early as 4 h post-contamination. The presence of metformin in these samples raises the possibility of metformin exposure through contamination, especially given the housing conditions at racetracks. The potential for exposure through contamination, resulting in adverse analytical findings, should be considered in the regulation of metformin in racehorses.