The equine industry is overrun with supplements. Some of the most common supplements are those that claim to increase the amount of red blood cells as this theoretically will increase oxygen to the cells and therefore increase the horse's speed. Dr. Ed Kane reports in DVM 360 that blood builders, or hematinics, usually include nutrients such as iron, copper, B12, thiamine, riboflavin, folic acid, and recombinant human erythropoietin. These products are reported to increase red blood cell production and hemoglobin in an effort to boost aerobic capacity in exercising horses but there is little evidence of their efficacy and safety.
The major component of many of these products is iron although iron is well regulated by the body and a deficiency would be rare in horses. If there is no deficiency, supplemental iron is unlikely to be effective at increasing the number of red blood cells, and products containing iron have shown no benefit. Giving a horse products such as human erythropoietin that actually do increase red blood cell production is unethical, potentially dangerous to the horse's health, and can cause serious side effects especially when used with other drugs. Although the erythropoietin increased red blood cell mass, there was also an increase in blood viscosity (thickening of the blood). The researchers studying the drug were concerned about getting the blood too thick, which is especially a concern on horses taking furosemide to prevent lung bleeding while running. Many people check a resting blood sample on a horse and want the red blood cell count to be higher. However, the horse maintains about 30% of its blood in the spleen, and it is released with exercise so a resting blood sample is not really indicative of the horse's actual number of red blood cells.