The New Mexico Racing Commission (NMRC) has suspended the use of the bronchodilator clenbuterol in Quarter Horses and Thoroughbreds racing that state though July 2016. The suspension went into effect on July 23.

Clenbuterol is the only Food and Drug Administration-approved for treating reversible bronchospasm in horses. The drug is commonly used to treat horses with inflammatory airway disease and recurrent airway obstruction (or heaves). It is also sometimes administered to racehorses during the racing season, however some researchers have questioned the side effects of this routine use.

Horse racing jurisdictions across the United States make their own rules about what substances can be lawfully administered to horses competing in races at their tracks. At its July 22 meeting, the NMRC suspended clenbuterol use, a letter posted on the commission’s website said. The suspension prohibits clenbuterol’s presence in all test samples collected from Thoroughbreds and Quarter Horses slated to race at any New Mexico track. The suspension also allows for an adjustment period between July 23 and Aug. 19, during which no action will be taken for the minimal presence of the drug found in post-race and out-of-competition testing.

Beginning in Aug. 20, all samples with any trace of clenbuterol will be considered a violation of NMRC rules and will be prosecuted as a medication violation, the letter said.

The American Quarter Horse Association (AQHA) has banned the use of clenbuterol in any AQHA race, beginning in January 2016.