The justices left in place an appeals court ruling that upheld the law and rejected claims that Congress gave too much power to the Horseracing Integrity and Safety Authority, the private entity that administers the rules.
Oklahoma, Louisiana and West Virginia sought to have the law struck down, joined by several racetracks.
The anti-doping program, which took effect in the spring of 2023, is an attempt to centralize the drug testing of racehorses and manage the results, as well as dole out uniform penalties to horses and trainers instead of the previous patchwork rules that varied from state to state.