Human EPO is bad for horses in a way that isn't bad for humans because it is a foreign substance to which they build antibodies, which can in turn attack their natural production of red blood cells. It can cause anemia and worse.

In horses or people, excess levels of red blood cells can inhibit, not improve, performance because all the oxygen-carrying capacity in the world won't do you any good if it can't circulate effectively. A too-high percentage of red blood cells causes blood to be sludgy and not circulate well. It can cause sudden death due to blood clots in the brain and heart. There have been many sudden deaths in athletes thought to have been caused by EPO use.

Horses are very different than people in that they store massive amounts of red blood cells in their spleens and dump varying amounts into circulation to meet the needs of exercise. A horse can go from 40 to 60 percent red blood cells in its blood in moments due to this factor. Giving a healthy horse EPO is thought by many equine specialists to be pretty stupid, since it has documented, potentially deadly, side effects and horses already have what is considered to be an excess of red blood cells on hand naturally.