The unpredictable nature of Ibuprofen has reared its head again with the Matthew Williams-trained Oregon Spirit returning a positive result to the banned substance.

Racing Victoria stewards have elected not to charge Williams with bringing a horse to the races with a banned substance in its system, satisfied the trainer took measures to ensure Oregon Spirit was free of Ibuprofen.

Oregon Spirit was scheduled to race at Flemington in December but was scratched at the barrier after rearing in the stalls and a subsequent urine sample detected Ibuprofen.

"This particular case is consistent with a number of others that stewards have been investigating which relate to the administration of Ibuprofen on horses conducted during a rehabilitation regime outside the care of their trainers," RV chief steward Terry Bailey said.

"This case highlights how complex the prolonged use of this substance is in racehorses because of its erratic and unpredictable clearance from their system."

Williams proactively had samples from the gelding tested by Racing Analytical Services and after three clear results decide to take him to the races.

Stewards were satisfied Williams did not administer Ibuprofen to Oregon Spirit, or any other horse in his stable, at any time.

Williams has told stewards his intention is to retire and rehome Oregon Spirit.

Other horses who have been caught up in the Ibuprofen saga include Signoff who is set to make his return to racing at Caulfield on Saturday in an 1800m-handicap for new trainer Patrick Ryan.