At that level of competition there is NO way the rider [sometimes owners are just on paper] doesn't know whats going on. The horse industry is @ a very sad stage that illegal practices are the norm.
2008 Olympics
Rodrigo Pessoa [Brazil/ShowJumper] Tested positive-fined
Courtney King Dye [United States/ Dressage] Tested positive-fined
Phillip Dutton [United States/ Eventing] Illegal use of weighted boots-fined
Capt. Mark Phillips [United States/Chef E' Quip] Knowledge of wrongful practices-fined, and there were multiple others.
It would be nice if this didn't happen, unfortunetly it is the norm. 1 magazine "Equestrian" is the publication for the United States Equestrian Association comes out monthly, in the back are fines and suspensions, that section is never less than 15-20 pages. And that is just 1 book,each month.
The bigger the stakes, the more people/ways they find to try and cheat.
Capsaicin is made from pepper juice- It has a numbing effect, sort of like mild lidacain. It can be purchased otc, one that comes to mind is Farnams Thera-Flex