Leading jockey Zac Purton was cleared to ride at Happy Valley on Wednesday night after a voluntary drug test but, in a separate matter, Jockey Club stewards withdrew two David Hall-trained runners from the meeting after positive tests for arsenic.
Stewards said Purton had submitted a urine sample on Monday for testing, at his own request, for the presence of any banned substances. On Wednesday afternoon, Dr Terence Wan See-ming, the club's chief analyst, reported a negative return.
Chinese media reports had claimed Purton was believed to have been spiked with GHB while drinking in a Lan Kwai Fong bar on Saturday night with his wife and several other jockeys, had later felt unwell, and was taken to Queen Mary Hospital to be checked over.
The jockey had not ridden trackwork for two days afterwards, but Purton said the rumours had greatly overplayed the matter.
"It's not unusual for me to be checked over in hospital. I have had to take care with my health since I had the kidney stones that put me in hospital this time last year," the Australian said on Wednesday.
"I had a few drinks on a night out on the weekend and this is not the first time that I've asked the Jockey Club to test me in the same sort of circumstances."
Meanwhile, the club's laboratory reported that a preliminary, unconfirmed analysis of the urine sample from David Hall-trained Hit A Home Run, second at Sha Tin on Sunday, showed arsenic levels above the permitted threshold and that pre-race urine tests of his scheduled Happy Valley runners Gold Tartini and Heroic Guru were showing arsenic as well.
Gold Tartini's levels had exceeded the 300 milligrams/millilitre threshold, while Heroic Guru's level had not exceeded the permissible level; however Hall requested both horses be withdrawn from their races when he was informed of the findings.
After inspecting Hall's stables and medication records, stewards became aware that both he and Peter Ho Leung "used extensively" certain supplements, and pre-race samples were taken from a number of Ho's Happy Valley runners as well.
The laboratory finding was that, whilst Ho's horses also had high arsenic readings, these readings had not exceeded the threshold levels and they were permitted to run.
The Jockey Club has also released Australian jockey Tye Angland from the remainder of his contract.
Angland was due to ride out the season and had announced he would not return next season; however, due to family reasons, he recently had to rush back to Australia.
At that time, the jockey had been given permission to put his riding on hold temporarily, but he has now applied for and been granted a release and will not return.
