The Hong Kong Jockey Club (HKJC) has determined that the banned substance zilpaterol found in tests taken from 12 racehorses trained by P. F. Yiu was the result of contaminated feed, and has determined that a second feed product used by trainers has similarly been contaminated.

Zilpaterol is a beta-2 agonist used to promote weight gain in livestock, but the drug is prohibited in racing. In North America, the Association of Racing Commissioners International classifies zilpaterol as a Class 3 drug with a Category A penalty.

Following the positives for horses trained by Yiu that had raced June 2 at Sha Tin Racecourse and at Happy Valley on June 5, stewards "immediately undertook an exhaustive testing regime of all products used by Mr. Yiu which may have been administered to horses trained by him."

Those tests revealed the presence of zilpaterol in feed used by Yiu and subsequent tests detected the prohibited substance in another feed used by trainers in Hong Kong. Neither feed manufacturer was identified