UC Davis study links metformin positives to stall contamination, Wong weighs legal action

A UC Davis contamination study may have given racing its clearest evidence yet that some metformin positives can come from a stall, not a syringe. In tests on six exercised Thoroughbreds, two mares and four geldings aged 3 to 5, horses exposed to urine-contaminated bedding tested positive after four hours and again at 24 hours, while four horses exposed to higher concentrations still tested positive seven days later.

That finding lands hard in a sport already wrestling with how to separate wrongdoing from exposure. The Horseracing Integrity and Safety Authority has already stayed nine pending metformin adjudications from late 2024 because of contamination concerns, and the Horseracing Integrity and Welfare Unit now has nine metformin cases pending from 2024-25. The science is pushing regulators toward a threshold-based model, a major shift from the way the sport treated the drug when any trace could trigger punishment.

In 2024, HISA’s anti-doping and medication control program treated any metformin finding above 0.5 ng/mL in blood and 1 ng/mL in urine as an adverse analytical finding. By November 2025, HISA had proposed a 4 ng/mL minimum reporting level for metformin in blood, a change still under review by the ADMC Committee and headed to the Federal Trade Commission if approved. The U.S. Trotting Association had already adopted a 5 ng/mL plasma no-effect threshold for metformin in harness racing in 2024, underscoring how quickly the industry is moving toward science-based cutoffs.

Jonathan Wong’s case is now the most visible test of what that shift means in practice. He was provisionally suspended in August 2023 after the B sample confirmed metformin in Heaven and Earth, the horse that won a maiden special weight at Horseshoe Indianapolis on June 1, 2023. HISA later imposed a two-year suspension, a $25,000 fine and an $8,000 assessment toward adjudication costs, with the penalty applied retroactively to July 1, 2023.

Arbitrator Nancy Holtz issued a 50-page ruling in the matter. Wong said in August 2023 that he was considering abandoning training and later said he would appeal.

The broader significance goes well beyond one trainer. If metformin can enter a horse through a contaminated stall and remain detectable for days, the central question is no longer only whether a positive exists, but whether the regulatory system can prove how it got there. That is the issue now facing HISA, HIWU and every horseman whose case may turn on a result that science says can be mimicked by the environment.