Champion trainer John Gosden has been fined £500 but cleared of intentional wrongdoing after one of his horses tested positive for ketamine.
The banned substance was found in a urine sample taken from Franconia after she won a race at Newbury in June 2020.
Ketamine is a powerful anaesthetic used in horses before surgical procedures, but is also a prevalent recreational drug used by humans.
A disciplinary panel accepted the most probable source was accidental contamination by the horse's groom, who admitted to using the drug recreationally, including the weekend before the race.
All parties accepted there had been no intentional administration of the drug.
Gosden was fined for taking most but not all "reasonable precautions" to prevent contamination of the drug, though the sanction was unusually low because of what the panel described as "exceptional circumstances".
Franconia was also disqualified from the race and the £17,296 of prize money will be redistributed.
Gosden represented himself at the hearing, explaining: "I haven't employed a lawyer because I've got nothing to hide."
But he said that the case had been an "alarm call" and was "not great for our stables".
Tomas Nolan, presenting the BHA's case, said that when investigators visited Gosden's yard last July about the positive case he had no explanation for the finding.
However, one of his grooms came forward to admit recreationally taking ketamine during weekends off, including on the weekend before the race in question, which Franconia won.
The staff member, whose identity was kept anonymous as he was described as "vulnerable", issued a statement saying that he had been careful to wash his clothes and hands, but residue of ketamine was in his wallet, which he had used on the trip to Newbury.
Traces of ketamine are thought to have got into Franconia's system when the groom placed the bit in her mouth before the race.
Nolan said an investigation by Dr Stuart Paine, an expert pharmacologist from Nottingham University, found this explanation was plausible.
Gosden said there was "great relief that we have managed to get to the source of the problem", but said the prevalence of recreational drug use in society made him think cases like this will become more frequent.
"I think what we have to face, all of us, is that it's not just here," he said.
"In every town, village and city there seems to be so much recreational drugs use going on that it is an ongoing, frightening risk.
"And if people aren't hygienic about it and careful about it, and with the sensitivity of testing, which I must say did impress me, it is likely we will see more cases of it. I don't know what you do about it."
While it was acknowledged that Gosden had taken some precautionary measures to prevent contamination, the BHA case was that more could have been done to ensure staff were aware of the specific risk of contamination if using prescription medication or recreational drugs.
The penalty structure for breaches of the rules regarding prohibited substances on racedays includes a range from a £1,000 fine up to a 10-year disqualification.
