(CNN) Suspended Russian tennis champ Maria Sharapova admits she made a "huge mistake" by taking a banned performance-enhancing drug called meldonium. But what is it? For what medical reasons would an athlete take it? And why is it banned? The drug, known as meldonium or mildronate, is not approved by the Food and Drug Administration in the United States, but it is prescribed elsewhere to help protect heart tissue by regulating metabolism. It has also been used by neurological clinics to treat patients who have asthenia -- an abnormal lack of energy. The substance had only recently come to the attention of the World Anti-Doping Agency, which started monitoring the use of the drug by athletes in 2015 and banned it outright beginning this year.Meldonium protects heart tissueIt was added to the list of anti-doping drugs in JanuaryThe normal course of treatment is days or weeks
_ was first prescribed the drug in early 2006, according to her attorney, after tests revealed that "she has some immune deficiency, abnormal EKG results, diabetes indicators, low magnesium and asthenia."