No one wants to lose a medal won at the Olympics because of a bad drug test on a horse. And it has happened...remember Athens in 2004?
Many horses are treated with legitimate medications, while others have been treated with medications that are not allowed for competition. One of the tricky aspects of preparing a horse for competition is knowing when the drug's traces will leave the horse's symptoms. Vets have guidelines to go by, but it is always a gamble.
Another thing that happens at competitions is that trace elements show up in drug tests that are a complete mystery. We hear all sorts of conjecture about substances that may be on feed tubs or that the horse absorbs through the skin from rubdowns or hoof treatments. There is also the implied threat of sabotage.
This year at the Olympics, horses are being offered optional drug tests upon their arrival. These tests have to be made by collecting urine within 12 hours of arrival in Hong Kong.
These drug tests are not legally binding, but riders and coaches and owners will know if a horse tests for a given substance. Some horses may be coming off medications and have plenty of time to be "clean" by the time that the competitions begin. An example would be lidocaine, a local anesthetic, or a steroid used to clear up a skin condition.
The system is called PAET, for Post-Arrival Elective Testing, and this is the first time it has been offered.