Trainer Paul Beshara has failed in his appeal against a six-month disqualification despite changing his plea to guilty.

Beshara was outed for giving Group One winner Happy Trails an injection of an unknown substance hours before the horse was due to race in the Dato Tan Chin Nam Stakes in September.

Counsel for Beshara, Paul Horvath, argued the case fell into the category of special circumstances and should therefore be reduced.

Judge John Nixon, sitting at the Victoria Civil & Administrative Tribunal, disagreed and dismissed the Adelaide trainer's appeal against the severity of the penalty, a mandatory six months under a rule introduced earlier this year.

Judge Nixon said Beshara had failed to establish the special circumstances which could lead him to reduce the penalty.

Beshara can return to training on April 21.

Happy Trails won the Group One Turnbull Stakes two weeks before Beshara's case was heard and finished a close second in the Cox Plate after being transferred to Byron Cozamanis.