NSW thoroughbred trainers have been warned not to give their racehorses an anti-inflammatory drug that is commonly found in a preparation used to treat tendon injuries.

Following a spate of positive swabs to ibuprofen in Victorian racing, Racing NSW stewards have declared ibuprofen off limits in the state's racing stables.

"Trainers are warned that it is difficult to make any reliable predictions about the clearance of ibuprofen from horses based on informative currently available," a stewards' statement said.

Horses which have undergone a prolonged rehabilitation program after a tendon injury were at most risk of returning a positive swab.

Stewards said they would work with trainers to facilitate an elective testing program.

Victorian racing has been hit with at least 20 positive swabs to ibuprofen.

The most prominent case involves Signoff, the Darren Weir-trained stayer who tested positive after a first-up win in December.