Newcastle trainer Darren Smith has lost his appeal against a 15-year disqualification for cobalt offences.

Last month, Judge David Amati of the Racing Appeals Tribunal rejected his appeal against conviction and on Monday upheld the 15-year ban.

Smith was convicted of 42 charges relating to high levels of cobalt found in horses from his Newcastle stable last year.

He was stood down in May 2014.

Smith lost his first appeal in front of the Racing NSW Appeal Panel and took his case to the highest racing tribunal.

Cobalt is a part of some vitamin supplements but is toxic in horses when given in high doses and a national threshold of 200mcg per litre of urine is in force.

Trainer Sam Kavanagh is the subject of an ongoing Racing NSW stewards inquiry sparked by elevated levels found in Midsummer Sun after his Gosford Cup win in January.

The Kavanagh case has spread to include Melbourne vet Tom Brennan named as the source of a substance called Vitamin Complex found to contain high levels of cobalt.