A cobalt threshold of 200 micrograms per litre of urine in racehorses has been introduced in NSW to take effect in the new year.

Racing NSW stewards have also warned trainers the administration of some vitamin supplements close to race day may bring the levels close to that level.

Cobalt is a naturally occurring trace element which may normally be present in horses at very low levels from their feed.

It is also present in the structure of vitamin B12 (cyanocobalamin). It is known that excessive amounts of cobalt have an effect on the blood system and it is banned by the World Anti Doping Agency.

Racing NSW is in the midst of an inquiry into cobalt levels of between 220mg/L to 6000 plus found in horses trained by Darren Smith at Newcastle.

The hearing is being conducted without the threshold on the basis it is a prohibited substance.

Racing NSW said the threshold had been determined following a national survey of the racing horse population which measured cobalt levels in normal racehorses during training and racing.

Trainers have been advised the administration, particularly by injection and on multiple occasions, of certain registered vitamin supplements close to racing may result in a level of cobalt in a subsequent sample that exceeds this threshold.