More than two dozen people, including the trainer of champion Maximum Security, were charged in what authorities described Monday as a widespread international scheme to drug horses to make them race faster.

Trainer Jason Servis, whose stable includes the 3-year-old champion, was charged with administering performance-enhancing drugs to that horse and others. Maximum Security crossed the finish line first at the 2019 Kentucky Derby before being disqualified for interference and has since won four of his five high-profile races.

The charges against trainers, veterinarians and others were detail...