According to reports, Sharapova was prescribed a drug name Mildronate, which is another name for Meldonium, by her family doctor for health issues in 2006 and she's has been taking it for the past 10 years.
Maria Sharapova winner of five grand slams and former no 1 Tennis champion has been provisionally suspended after she failed a drug test at the Australian Open. She tested positive for meldonium, a drug that is the banned by World Anti-Doping Agency (WADA).
According to reports, Sharapova was prescribed a drug name mildronate, which is another name for meldonium, by her family doctor for health issues in 2006 and she’s has been taking it for the past 10 years. The drug was not on WADA’s list of prohibited drugs, but rules changed on January 1, and meldonium became a prohibited drug, which she was not aware of.
Meldonium or mildronate is an anti-ischemic drug which means it promotes continuous blood supply to the heart and is widely used in treating heart disease and central nervous system function. 
Apart from its anti-ischemic properties, it is known to improve endurance in athletes, improve their rehabilitation after exercise, provide protection against stress and also enhance the activity of the central nervous system. A study published in December 2015 points that Mildronate is not on the WADA’s list of prohibited drugs and points out that athletes use it not merely for medicinal reasons. It also encouraged adopting different testing methods for the drug owing to its rampant misuse. The study says that mildronate was added WADA’s monitoring program in 2015. 
Sharapova had given the samples for testing on January 26 during the Australian Open, which tested positive for meldonium and she was banned on March 2. She says that she had received a mail from WADA on December 22 with a link to all the banned substances and she did not click on the link. She says that she has made a huge mistake and that she has let her fans down and also the sport that she loves so dearly. In a report she also said that she doesn’t wish to end her career like this!
1. Sjakste N, Gutcaits A, Kalvinsh I. Mildronate: an antiischemic drug for neurological indications. CNS Drug Rev. 2005 Summer;11(2):151-68. Review. PubMed PMID: 16007237.
2. Görgens C, Guddat S, Dib J, Geyer H, Schänzer W, Thevis M. Mildronate(Meldonium) in professional sports – monitoring doping control urine samples using hydrophilic interaction liquid chromatography – high resolution/high accuracy mass spectrometry. Drug Test Anal. 2015 Dec;7(11-12):973-9. doi: 10.1002/dta.1788. Epub 2015 Apr 5. PubMed PMID: 25847280.