Lisa Cropp was stood down from her four rides at Sale on Wednesday after returning an irregularity to the prohibited substance Zopiclone in a urine sample.
Zopiclone is a proscribed medication used in the treatment of insomnia.
Racing Victoria Integrity Services executive Dayle Brown said at this stage it was an irregularity only.
But Cropp would not be allowed to ride until a stewards’ inquiry was held into the matter.
Cropp had been booked for five rides at Warrnambool on Thursday, five at Mornington on Friday and four at Moonee Valley on Friday night.