More than two dozen people, including the trainer of champion Maximum Security, have been charged in what authorities describe as a widespread international scheme to drug horses.

Trainer Jason Servis, whose stable includes Maximum Security, winner of the $20 million Saudi Cup, has been charged with administering performance-enhancing drugs to that horse and others.

Maximum Security crossed the line first in the 2019 Kentucky Derby but was disqualified for interference and has since won four of his five high-profile races.

The charges against trainers, veterinarians and others are detailed in four indictments unveiled in Manhattan federal court.

Charges brought against the 27 people include drug adulteration and mis-branding conspiracy.

Performance-enhancing drugs "were given to racehorses in an effort to increase their performance beyond their natural abilities," William F Sweeney Jr, assistant director in charge of the FBI New York Office, said at a news conference.

"What actually happened to the horses amounted to nothing less than abuse.

"They experienced cardiac issues, overexertion leading to leg fractures, increased risk of injury, and, in some cases, death.

"Conversely, the human being involved in the scheme continued to line their purses as they manipulated this multi-billion-dollar horse racing industry across the globe."

Authorities say the drugs can cause horses to overexert themselves, leading to heart issues or death.

According to the indictments, other drugs used to deaden a horse's sensitivity to pain to improve the horse's performance could also lead to leg fractures.

Authorities said participants in the fraud - affecting races in New York, New Jersey, Florida, Ohio, Kentucky and the United Arab Emirates - misled federal and state regulators, US Customs and Border Protection agents, various state horse racing regulators and the betting public.

National Thoroughbred Racing Association president and chief executive Alex Waldrop called the charges "abhorrent".

"There is no place in our sport for individuals who treat horses with disregard for their well-being or who undermine the integrity of our competition for personal gain," Waldrop said.

Federal authorities searched barns in Florida and a manufacturing facility in Kentucky.

In the indictment, Servis is charged with giving Maximum Security a performance-enhancing drug called SGF-1000, recommending it to another trainer, and conspiring with a veterinarian to make it look like a false positive for another substance. The other trainer, Jorge Navarro, is among those charged.

Servis is alleged to have given performance-enhancing drugs to "virtually all the racehorses under his control".

"The charges in this indictment result from a widespread, corrupt scheme by racehorse trainers, veterinarians, PED (performance-enhancing drug) distributors and others to manufacture, distribute and receive adulterated and mis-branded PEDs and to secretly administer those PEDs to racehorses under scheme participants' control," an indictment reads.

"A sad day for racing but a long time coming," trainer Graham Motion tweeted. "A good day for those who try to play by the rules, we will all be better for it."

One of Navarro's horses, X Y Jet, who is named in one of the indictments as a recipient of performance-enhancing drugs, earned more than $3 million in winnings and died of an apparent heart attack in January.