The California Horse Racing Board voted on Thursday to restrict the use of the bronchodilator clenbuterol in the state this summer, extending the suggested withdrawal time from four days to 21.The new guideline will take effect with the start of the Del Mar meeting, on July 18.Currently, clenbuterol is allowed to appear in post-race tests at trace levels – five nanograms of urine or 25 picograms of blood. Those limits will be lowered by July 18, although the new levels have yet to be determined, according to professor Scott Stanley of the University of California-Davis School of Veterinary Medicine. He said the university is conducting a study led by assistant professor Heather Knych that intends to determine the new levels in June or July.The seven-person racing board voted unanimously for the restricted use of clenbuterol.The restricted use will be in effect for one year and applies to all breeds racing in the state. Since last October, clenbuterol has been banned for use in Quarter Horses at Los Alamitos after a series of overages occurred at that track.The Los Alamitos ban was enacted after forms of clenbuterol not approved by the United States Food and Drug Administration were found being used on Quarter Horses, primarily as a muscle-building agent.Dr. Rick Arthur, the racing board’s equine medical director, told the racing board on Thursday that owners and trainers spend $5 million to $7 million on clenbuterol annually.The shift to a 21-day withdrawal had the approval of the Thoroughbred Owners of California, according to president Lou Raffetto. “We want to ensure that it’s used for therapeutic purposes but at the same time not have a negative effect on California racing,” Raffetto told the board.