Heard Marquez drank his own **** not only during training camp, but also before the fight. Canelo eats a "special" carne asada taco before his ring walk.
Caffeine was completely banned from sports (Olympics at least, I think) for some 20 years and was just recently lifted. Nowadays it's not completely allowed, I think there are still governing bodies like WADA that limits the amount of caffeine detected in your pee. Not sure that includes boxing though, but certainly endurance sports.