A substance derived from chilli peppers and used to stop horses from chewing on fences has produced positive swabs in harness racing, prompting Racing NSW to issue a warning to thoroughbred trainers.

Capsaicin is banned because of its effects on the nervous, cardiovascular and digestive systems and is also categorised as an analgesic and vasodilator.

It is the active component of chilli peppers which belong to the capsicum family.

Capsaicin is an irritant when applied externally and produces a sensation of burning in any tissue with which it comes into contact.

Racing NSW chief steward Ray Murrihy said Capsaicin was the active component in a preparation called `Stop Crib' which is a brushable paste applied to fences and rugs to stop chewing and crib biting.

He said there were also anecdotal reports of capsaicin arising from the administration of non-proprietary drenches that have been touted for pre-race use in horses.