Maria Sharapova told a news conference Monday that she failed a doping test during the Australian Open in January.
She was recently notified of the failed test in a letter from the International Tennis Federation.
The 28-year-old Russian tennis star said she has been taking the medicine mildronate, prescribed by a family doctor, since 2006 for a magnesium deficiency. The substance, also known as meldonium, was added to the banned list this year.
According to a press release from the Tennis Anti-Doping Program, the positive sample was collected on Jan. 26 and Sharapova was charged with the violation on March 2. She will be provisionally suspended, effective March 12, "pending determination of the case," it said.
Sharapova takes responsibility
"I have to take full responsibility of it because it's my body and it's what I put in my body," said Sharapova, a five-time Grand Slam champion.
Sharapova said she received an email from the World Anti-Doping Agency on Dec. 22, 2015, with the updated list of prohibited substances but did not look at it.
"I've let my fans down. I've let this sport down that I've been playing since the age of four, that I love so deeply," said Sharapova.
Meldonium is a Latvian-manufactured drug popular in former Soviet Union countries for fighting heart disease.
Several athletes have tested positive for the drug since it became illegal in January, including two Ukrainian biathletes and Russian cyclist Eduard Vorganov.
Earlier Monday, Russia's Ekaterina Bobrova, a European champion ice dancer, told local media she had tested positive for meldonium. Two high-profile runners were also caught with the drug: Ethiopian runner Endeshaw Negesse, winner of the 2015 Tokyo Marathon and Swedesh 1,500-metre runner Abeba Aregawi.
Infraction normally carries 1-year suspension
WADA president Craig Reedie told The Associated Press an athlete found guilty of using meldonium would usually face a one-year suspension. The organization's spokesman, Ben Nichols, said it "will refrain from commenting further until a decision has been issued by the ITF."
Sharapova has won 35 singles titles since breaking onto the world tennis scene in 2004 by winning Wimbledon at the age of 17. She is ranked seventh in the WTA.
Sharapova, currently sidelined with a forearm injury, hasn't competed since losing to Serena Williams in the quarter-finals of the Australian Open.
The Los Angeles press conference was hastily organized and announced Sunday, saying Sharapova would make a "major announcement." Many suspected she would announce her retirement.
Sharapova is believed to be the world's highest-paid female athlete, and is involved in multiple off-court business ventures.
'Angry' Capriati sounds off
On her Twitter account Monday, former tennis star Jennifer Capriati said she was "extremely angry and disappointed" in Sharapova.
Im extremely angry and disappointed. I had to lose my career and never opted to cheat no matter what.i had to throw in the towel and suffer— @JenCapriati
i didn't have the high priced team of drs that found a way for me to cheat and get around the system and wait for science to catch up— @JenCapriati
The 39-year-old Capriati won three major titles and an Olympic gold medal after starting her pro career as a 13-year-old prodigy. She reached the top 10 when she was 14, becoming the youngest player to do so.
A back injury plagued the American's final professional season in 2004, and Capriati has said she was forced to retire earlier than she had hoped.