Recent studies have shown a link between the use of the hormone replacement product Prempro and an increased incidence of breast cancer, heart attack, and stroke. Prempro, along with other popular hormone replacement drugs, are derived from the urine of pregnant mares. Bio-identical hormone therapies, on the other hand, are derived from plant sources such as yams and soybeans. They precisely match the hormones naturally found in the human body. While anti-aging centers and pharmaceutical corporations claim that bio-identical hormone drugs do not increase the risks of cancer or other health threats as other hormone treatments do, several prominent medical organizations warn that bioidentical hormones are just as dangerous. The Endocrine Society, an international, professional medical organization serving the field of hormone study, issued a statement that bioidentical hormones carry the same risks as the famously dangerous animal-derived hormones. The Mayo Clinic states that there is no evidence that bioidentical hormones are a safe alternative to conventional hormone treatments. The United States Food and Drug administration has sent letters to pharmacy companies that produce bioidentical hormone drugs warning them that the claims the drug companies were making touting the relative safety and effectiveness of these drugs were misleading to women and healthcare professionals and not supported by medical evidence. The FDA is willing, however, to allow licensed healthcare practitioners to prescribe bioidentical hormone treatments in cases where conventional medicines fail to meet patients' needs.