Keep in mind that with a drugged horse, it's already racing beyond what his own confirmation and physiology would normally allow. When you throw in extreme and excessive urging on top of that, the stressors on the horse are well beyond it's natural physical limit, and the potential for trouble is elevated. Certainly not sustainable for the long term.
Think of putting a Hemi into a Yugo. It may work for an indeterminate amount of time, but considering what that particular structure was intended for - something will eventually fail.
