Maria Sharapova announced on Monday that she failed a drug test during this year's Australian Open.

The Russian-born tennis star said she received a letter from the International Tennis Federation several days ago informing her that she had failed the test for meldonium during the Australian Open in January.

"I did fail the test," Sharapova told reporters at a hastily arranged press conference in Los Angeles. "And I take full responsibility for it."

The five-time Grand Slam champion said that she had been legally taking the drug, meldonium (also known as mildronate), as prescribed by a family doctor for a decade to combat a variety of illnesses while on tour.

What is meldonium?

According to the National Institutes of Health, the anti-ischemic drug meldonium "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 the Guardian newspaper, it is manufactured in Latvia and not approved by the Food and Drug Administration for use in the United States.

Why is it a banned substance?

Due to "the growing body of evidence" concerning its misuse by athletes, the substance was added to the World Anti-Doping Agency's monitoring program in 2015. WADA scrutinized the global athlete usage of the mildronate — which was which was not previously prohibited — to determine if the rates of use indicated any potential performance-enhancing concerns, and it wound up on its banned list on Jan. 1.

Sharapova said she had received an email from the ITF in December with a link to an updated list that included the newly banned substances, but admitted she "did not click on that link."

In a statement released following Sharapova's press conference, WADA said the drug was added to the list of prohibited substances in 2016 "because of evidence of its use by athletes with the intention of enhancing performance."

The 28-year-old, who splits time between her homes in Bradenton, Fla., and Manhattan Beach, Calif., insisted she had been prescribed mildronate to treat chronic illnesses, a magnesium deficiency and family history of diabetes.

What other substances are on the banned list?

In short, a lot.

Who else has been caught using it?

It turns out, quite a few athletes.

Russian cyclist Eduard Vorganov was suspended this month after he tested positive for the drug, which boosts oxygen supply to the blood and tissues in the body.

Ukrainian biathletes Artem Tyshchenko and Olga Abramova as well as Ethiopian-born runners Abeba Aregawi and Endeshaw Negesse each tested positive for meldonium in recent weeks.

Abramova was provisionally suspended in early February after testing positive for the metabolism booster. According to the AP, failed a test on Jan. 10, nine days after the drug was added to the World Anti-Doping Agency’s banned list.

The IAAF announced last week that Aregawi, who now competes for Sweden, is a former 1,500-meter world champion, failed an out-of-competition doping test.

And on Monday, fellow Russian athlete and Olympic gold medallist figure skater Ekaterina Bobrova also admitted to testing positive for the drug.

What does this mean for Maria?

Tennis officials have yet to indicate what kind of punishment Sharapova will face, but a lengthy suspension and hefty fine are possible, even likely.

In fact, it's already been pricey for the world's highest-paid female athlete. Nike, Swiss watchmaker TAG Heuer and Porsche — three of Sharapova's most prominent sponsors — cut ties with the statuesque Russian within 24 hours of her announcement.

What does the tennis world think?

Serena Williams, who defeated Sharapova at the Australian Open on the day the test was administered, said her rival showed "a lot of courage" for accepting responsibility for the failed test.

"Most people were surprised and shocked, but happy that she was upfront and very honest," Williams said Tuesday.

Nick Bollettieri, who is credited with grooming a teenage Sharapova at his Florida tennis academy, told the BBC he was shocked because "she has always been above board in everything."

But Bollettieri said he believes Sharapova made a "very honest mistake."

"She said she took these for many, many years and then didn't read the memorandum that came out," he said. "I don't think that Maria Sharapova would continue doing something, especially being in the limelight, if there was something she knew about."

Former American tennis star Jennifer Capriati was not as understanding.

"[I'm] extremely angry and disappointed," Capriati, whose career was plagued by injuries and off-court issues including a stint in drug rehab, tweeted. "I had to lose my career and never opted to cheat no matter what. [I] had to throw in the towel and suffer."

Capriati continued: "I didn't have the high priced team of [doctors] that found a way for me to cheat and get around the system and wait for science to catch up." She subsequently deleted the tweets.

What's next?

Sharapova — who stoked rumors of a possible retirement by calling for Monday's press conference following a withdrawal from an upcoming tournament in Indian Wells, Calif., due to injury — said she isn't sure what kind of penalty she'll receive, but hopes to return to the tour.

"I don't want to end my career this way," she said. "I really hope I will be given another chance to play this game."

Sharapova added: "I know many of you thought I would be retiring today, and announcing my retirement. But if I ever was going to announce my retirement, it would probably not be in a downtown Los Angeles hotel with a fairly ugly carpet."