Queensland trainers have been warned to be vigilant regarding the withholding period of a sedative used on horses during surgery after a spate of positive swabs.

In the past two weeks stewards have fined four trainers after the prohibited substance 4-Hydroxy xylazine was detected in urine samples taken from their horses.

Mark Palmer, Brad Herne, Ben Hull and Ben Currie were all fined $3000 and their horses disqualified.

Palmer's fine involved Star Painter at Gatton on April 20, Hull's from Mighty Mittere at the Sunshine Coast on April 30, and Herne's for Je Suis Prest at the Sunshine Coast on March 19.

The Currie-trained Smooch was found to have the drug in her urine after running at the Sunshine Coast on March 19.

In each case stewards heard a sedative which contained xylazine had been administered during a minor procedure and the trainers believed the horses started after the time had elapsed for the drug to still be in their systems.

Stewards warned trainers should make certain of the withholding period after the drug had been used.