Queensland Jockeys Association boss Glen Prentice has warned riders to seek an exemption from stewards if they are using medication after a third jockey was suspended for returning a positive result to a sleeping pill substance.

Jockey Justin Stanley is the latest rider to be suspended after taking half a sleeping pill which contained Temazepam the night before a race meeting at Cairns on September 13.

He is the third Queensland jockey or track rider to be suspended after returning a positive to a prohibited substance commonly found in sleeping pills and it is understood there is at least one other case pending.

Prentice said substances such as Temazepam could show up in urine tests many hours after it had been ingested as was the case with Stanley.

"Obviously a lot of people in racing, for that matter in general, use sleeping medication because of the odd hours they work," he said.

"The thing is you can apply to stewards for an exemption and it is something jockeys and track work riders should consider.

"As for a three-week suspension, it does appear steep compared with some other offences and it is a matter for later discussion."

Stewards completed an inquiry this week in which Stanley, the state's leading rider last season, was charged after a urine sample he provided showed traces of Temazepam.

The inquiry heard Stanley explained the medication had been prescribed by his doctor and he took it infrequently.

Stewards found Stanley had not made the appropriate application for an exemption to take the medication.

Stanley then pleaded guilty to a charge of providing a urine sample which contained a prohibited substance.

Stewards imposed a three-week suspension which Stanley has already served because it was back-dated.

Stanley is currently sidelined with a wrist injury and hopes to be back riding track work in January.