Horses exposed to urine contaminated with the illegal substance metformin in their stalls can test positive for the drug as far out as seven days from the time of exposure, according to a study conducted by the School of Veterinary Medicine at the University of California-Davis. The results of the study are likely to inform an ongoing process on how to regulate metformin, an active ingredient in a widely prescribed diabetes medication for humans. The drug is a banned substance, but the Horseracing Integrity and Safety Authority stayed nine pending adjudications for positives of the drug in late 2024 due to concerns about accidental contamination. In the study, six horses that were placed in stalls with the urine-contaminated bedding tested positive for the drug at four hours after exposure and at 24 hours after exposure. Four of the horses exposed to higher concentrations of the drug tested posted seven days after being placed in the stalls, according to the study. Horses commonly ingest strands of their bedding. HISA has told trainers to insist that their staff refrain from urinating in stalls, but several trainers facing adjudications for the drug have contended that the positives were from accidental contamination. The study’s authors said that the results support a regulatory strategy that would establish a threshold for the concentration of the drug in urine samples to separate intentional administrations of the drug from inadvertent exposures. The rule that was stayed in 2024 treated any finding of the drug as a positive, with a recommended penalty of a two-year suspension. “Data from this contamination study, combined with concentrations from horses knowingly administered metformin, can be used to establish a screening limit that accounts for inadvertent exposure through contamination, thus reducing the likelihood of [an adverse analytical finding] due to inadvertent exposure,” the authors wrote. The pharmacological impact of metformin has not been studied in horses other than older broodmares, but anecdotal evidence collected by regulators and investigators has suggested the drug could be used to increase a horse’s metabolic rate. In November of last year, HISA proposed a rule establishing a threshold level for metformin in blood. The level is currently being evaluated by HISA’s Anti-Drug Medication Control Committee. Any proposed rule would need to be forwarded to the Federal Trade Commission, HISA’s governing body, for final approval, a process that generally takes several months. :: Want to learn more about handicapping and wagering? Check out DRF's Handicapping 101 and Wagering 101 pages.