The Ultimate Fighting Championship (UFC) is filled with some of the world's toughest and most talented fighters. As the world's most famous mixed martial arts promotion, this perhaps comes as no surprise. However, the UFC is no different than any other promotion of the sport. Sometimes even competitors on the highest level are caught with performance enhancing drugs. Consequently, the UFC has joined forced with Olympic drug testers United States Anti-Doping Agency (USADA) to crack down on PED usage. As reported by MMA Fighting's Shaheen Al-Shatti, the top five substances that UFC competitors search for are cortisone, creatine, cannabis, cocaine and marijuana. For more MMA news click here.