Metformin, a banned drug in horse racing that is an ingredient in commonly prescribed medications to treat diabetes in humans, will be subjected to a higher threshold level before being called as a positive under a proposed new rule for the sport, according to the Horseracing Integrity and Safety Authority.
Under the proposed new rule, which will be published for public comment “in the coming days” and then face an approval process that typically lasts several months, metformin will not be treated as a violation unless it appears in a blood sample at a concentration exceeding 4 nanograms per milliliter, HISA said. The previous standard, which was devised to standardize results at U.S. testing laboratories, was 0.5 nanograms per milliliter.
The new standard is based on an initial review of the available science on the drug and a three-part study recently completed by Dr. Heather Knych at the University of California, which HISA said has been accepted for publication in Drug Testing and Analysis.
A scientific advisory committee at the Racing Medication and Testing Consortium approved the 4.0 nanogram recommendation as a marker of “exposure to metformin due to intentional administration and minimizes the possibility of a reported [Adverse Analytical Finding] due to inadvertent exposure,” HISA said.
The committee also recommended that metformin results should be based solely on blood tests, due to “erratic elimination behavior in urine and an inconsistent relationship between blood and urine concentrations.”
In the summer of 2024, HISA stayed all pending adjudications of metformin cases after horsemen continued to raise concerns about accidental contaminations. A total of nine cases were affected by the stay.
At that time, two trainers had already received lengthy suspensions for positives, including Jonathan Wong, who received a two-year suspension after an arbitrator ruled that he made “demonstrably false” and “untruthful” statements during a hearing on his appeal.
Metformin cases first began cropping up in Thoroughbred racing in 2020, prior to the advent of HISA and its drug-enforcement arm, the Horseracing Integrity & Welfare Unit. Trainers with positives at that time argued that the drug was the result of accidental contamination and received relatively light penalties, despite metformin being a banned drug under pre-HISA regulatory schemes.
HISA has said that its agents have received tips saying the drug was being used to “enhance performance,” perhaps by stimulating a horse’s metabolism. Aside from a study examining weight-loss in older mares, the biological effects of the drug on horses had not been the subject of a study until Dr. Knych’s recent research.
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HISA said in a release that under the new rule, metformin would remain a banned drug. HISA also noted that since the start of 2025, no laboratories have reported a positive for metformin even while using the stricter standard.
The nine pending cases will remain stayed while the rule is being reviewed for final approval by the Federal Trade Commission, HISA’s overseer. If the new threshold is approved, those cases in which the concentration of metformin was in excess of the 4.0 nanogram limit will be treated as violations, while the others will be withdrawn, according to Alexis Ravit, a spokesperson for the Horseracing Integrity and Welfare Unit.
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