The chances of Toowoomba's Ben Currie claiming his first Queensland trainers's premiership have taken a blow, with one of his winners disqualified.

Stewards opened an inquiry after Smooch returned a positive test to marker 4-Hydroxy Xylazine after winning at the Sunshine Coast on March 19.

Chief steward Allan Reardon said the drug could be used to produce sedation and analgesia in horses.

Currie was charged with bringing a horse to race with a prohibited substance.

He pleaded guilty and was fined $3000, with Smooch disqualified and the race going to Dream of Ascot.

It means Currie's statewide total is reduced from 100-1/2 to 99-1/2 winners which is 1-1/2 in front of Tony Gollan.