A Louisiana veterinarian has been charged with engaging in a scheme to influence the outcome of horse races by illegally treating the animals with a synthetic version of a drug known as "frog juice."

The federal indictment accuses the veterinarian, Kyle James Hebert, of providing trainers with syringes of dermorphin to inject the painkiller in at least four horses that competed at Louisiana racetracks. The indictment returned Thursday by a grand jury in the Western District of Louisiana says Hebert told trainers that the mislabeled drug would make the horses "focus" and run faster.

Dermorphin, an opioid roughly 30 times more potent than morphine, is naturally secreted by tree frogs native to South America. The U.S. Food and Drug Administration hasn't approved any drug containing dermorphin for use in humans or animals.

Hebert's company, Southern Equine Sports Medicine, operated veterinary clinics in Lake Charles and Sunset. The indictment charges him and an Omaha, Nebraska-based company, Kohll's Pharmacy & Healthcare Inc., with conspiracy.

Hebert is licensed to practice veterinary medicine at racetracks by the Louisiana State Racing Commission. In 2012, the commission sanctioned nine trainers whose horses tested positive for dermorphin.

One of the sanctioned trainers told the commission that Hebert gave drugs to his horses and had claimed they were "human herbs that would boost metabolism and help them breathe a bit," according to a Times-Picayune report in September 2012.

Charles Gardiner, the commission's executive director, said he believes the indictment secured Thursday by U.S. Attorney Stephanie Finley's office marks the first criminal charges over the use of dermorphin at Louisiana racetracks.

Gardiner said 11 horses linked to the nine trainers tested positive for the drug. He called it "the most serious offenses in the history of Louisiana racing."

"It endangered the horses, it affected the outcome of races and it defrauded the wagering public," Gardiner said.

Hebert and the Nebraska company didn't immediately respond to requests for comment on Friday.

The indictment says Hebert purchased approximately $25,000 worth of the mislabeled drug from Kohll's between November 2010 and May 2012. Kohll's invoices falsely identified the drug as "d-peptide," a fictitious name, according to the indictment.

The names of the racetracks where Hebert and his employees allegedly provided trainers with dermorphin aren't disclosed in the indictment.

Hebert also is accused of falsifying invoices for his veterinary services to conceal that the drugs were provided to trainers on race days.

Racing commission regulations prohibit anyone from injecting any substance into a horse within four hours of a race. The indictment says Hebert and his employees instructed trainers to perform injections one hour before race time.

Steven Barker, a professor emeritus from Louisiana State University's School of Veterinary Medicine, oversaw lab tests for dermorphin on samples collected for racing regulators in both Louisiana and Oklahoma.

Barker said the use and concealment of performing-enhancing drugs has always been a "cat and mouse game" in the horse racing industry. But its rate of positive drug tests is low compared to "human athletic endeavors," he said.

"The actual number of cases where there appears to be an obvious attempt to gain an advantage is very small," he said.