The British Horseracing Authority is looking into claims that the Mick Easterby-trained Ladies First was doped at Newcastle last month.
Successful at York 12 days previously, Ladies First was sent off the 6-4 favourite to win again but could only finish eighth of nine under apprentice jockey Scott McCullugh.
The filly failed a routine test, returning positive for a beta-blocker - which slows down the heart rate.
Assistant trainer David Easterby said: "We've been told that she failed a test for a beta-blocker.
"At the time her performance had us all flummoxed. We couldn't find anything wrong with her - it had us all stumped.
"We trust our staff implicitly - you'd also like to think that nobody who works with horses on a daily basis would be involved, because they would know the risks of doing something like this."
In a statement, the BHA said: "The BHA does not comment on investigations or speculation surrounding possible investigations.
"Should anyone have any concern or information regarding any integrity matters in British racing, we would ask them to contact firstname.lastname@example.org or call our anonymous 'Racestraight' reporting line on 0800 085 2580."