EPO (erythropoietin) is a hormone produced by the kidney that stimulates the bone marrow into producing more red blood cells. In theory, the greater the RBC mass, the greater the oxygen-carrying capacity and hence its use in endurance sports. The main problem is that with an increase in Hct, we get a corresponding decrease in viscosity, and the blood will tend to sludge and clog the smaller blood vessels. Most of the deaths attributed to use of this drug have been associated with dehydration (mostly cyclists) because their decreased water content, combined with the decreased viscosity, caused their blood cells clog the arteries and cause strokes. I think this would be a poor choice in bodybuilding because of the compounds atheletes are already using. #1 many AAS already cause a marked increase in RBC production (notably Deca and test which we sometimes use to treat anemia); and #2 diuretics which are being used at competition time to lose water. It's rumored in medical circles that it was the EPO sludge brought on by dehydration that killed Andreas Muntzer. I can see a theoretical benefit in training as a super Nitro-like O2 delivery system, but an absolute no-no for competition use.