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.
Sharapova's drug sample was collected on January 26.
How would meldonium help an athlete?
The drug "demonstrates an increase in endurance performance of athletes, improved rehabilitation after exercise, protection against stress, and enhanced activations of central nervous system functions," according to a study last year in the journal Drug Testing and Analysis
Side effects can include allergic reactions, indigestion, increased heart rate and changes in blood pressure, according to the drug's prescribing label
Meldonium is among a class of banned drugs called hormone and metabolic modulators that account for about 5% of doping in sports, according to a 2014 study
by the international doping agency.
Banned drug modulators have been used by cyclists, weightlifters, boxers, wrestlers, skiers and other athletes from more than 25 countries including Russia, Poland, Columbia, Belgium and Germany.
The agency said it added meldonium to its list of modulators "because of evidence of its use by athletes with the intention of enhancing performance."
Why was Sharapova taking it?
In an email to CNN, Sharapova's attorney said the tennis player took 500mg of mildronate on a regular basis for 10 years and that she was unaware the drug had another name and was banned.
Sharapova 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."
The manufacturer of mildronate, Grindex pharmaceuticals
, says the drug is "widely used for the treatment of different heart and vascular diseases." It notes the substance is also used to improve "work capacity" in healthy people who are overloaded physically or mentally or who are in a rehabilitation period.
The normal course of treatment, according to the drug's prescribing label
, is a matter of days to weeks.
It is not known why Sharapova's doctor continued to prescribe the drug for years.