(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.

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

Photos: Maria Sharapova: Richest female athlete of past decade Sharapova, age 14, plays her first professional tournament in March 2002. She defeated Brie Rippner at the Pacific Life Open in Indian Wells, California. Hide Caption 1 of 11 Photos: Maria Sharapova: Richest female athlete of past decade Sharapova holds up her trophy after she won Wimbledon in July 2004. The 17-year-old defeated Serena Williams in the final for her first Grand Slam title. Hide Caption 2 of 11 Photos: Maria Sharapova: Richest female athlete of past decade Sharapova serves during the 2006 U.S. Open semifinals. She advanced to beat Justine Henin in the final. It was her second Grand Slam title. Hide Caption 3 of 11 Photos: Maria Sharapova: Richest female athlete of past decade Sharapova is given a painting from the Children of Chernobyl Foundation in July 2007. That year, she became a U.N. global ambassador focused on the Chernobyl Recovery and Development Program. Hide Caption 4 of 11 Photos: Maria Sharapova: Richest female athlete of past decade Sharapova won her third major title at the 2008 Australian Open. Hide Caption 5 of 11 Photos: Maria Sharapova: Richest female athlete of past decade Off the court, Sharapova has become a fashion icon and a popular spokeswoman for many major companies. Her endorsements have included Nike, Gatorade, Canon and Cole Haan. Hide Caption 6 of 11 Photos: Maria Sharapova: Richest female athlete of past decade Sharapova falls to her knees after winning the 2012 French Open to complete the career Grand Slam. She also won the French Open in 2014. Hide Caption 7 of 11 Photos: Maria Sharapova: Richest female athlete of past decade Sharapova, left, at the London 2012 Olympic medal ceremony at Wimbledon with Serena Williams, center, and Victoria Azarenka. Sharapova finished with the silver after losing to Williams in the final. Hide Caption 8 of 11 Photos: Maria Sharapova: Richest female athlete of past decade Sharapova carries the Olympic torch in Sochi, Russia, during the opening ceremony for the 2014 Winter Olympics. Hide Caption 9 of 11 Photos: Maria Sharapova: Richest female athlete of past decade A shopper in Shanghai, China, walks past a Nike advertisement featuring Sharapova. Hide Caption 10 of 11 Photos: Maria Sharapova: Richest female athlete of past decade Sharapova signs autographs after winning a match at last year's Australian Open. She later announced she had tested positive for banned drug meldonium and was banned for two years, later reduced to 15 months. Hide Caption 11 of 11

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.

Photos: Athletes and doping in the era of PEDs Photos: Athletes and doping in the era of PEDs Maria Sharapova, a five-time Grand Slam champion and the world's highest-paid female athlete, admitted that she failed a drug test at the Australian Open in January. She tested positive for meldonium, a recently banned substance that she said she had taken since 2006 for health issues. She will be provisionally banned by the International Tennis Federation on March 12. Click through the gallery to see other athletes accused of using drugs to boost their careers. Hide Caption 1 of 17 Photos: Athletes and doping in the era of PEDs In February, New York Mets pitcher Jenrry Mejia became the first player to be permanently suspended by Major League Baseball after he tested positive for a performance-enhancing substance. MLB said Mejia tested positive for boldenone, an anabolic steroid that athletes use to increase muscle mass. Hide Caption 2 of 17 Photos: Athletes and doping in the era of PEDs New York Yankees slugger Alex Rodriguez confessed to using performance-enhancing drugs in a meeting with the Drug Enforcement Administration in January 2014. Rodriguez told DEA investigators that he had used banned substances, including testosterone cream, testosterone gummies, and human growth hormone, between late 2010 and October 2012. He was suspended for the entire 2014 season. Hide Caption 3 of 17 Photos: Athletes and doping in the era of PEDs Olympic sprinter Tyson Gay was banned for one year after he tested positive for a prohibited anabolic steroid in 2013. The four-time U.S. champion in the 100 meters received a reduced punishment from the two-year suspension standard for cooperating with authorities. The 4x100 relay team he was on was stripped of the silver medal it won in the 2012 Olympics. Gay returned to racing in 2014. Hide Caption 4 of 17 Photos: Athletes and doping in the era of PEDs Ryan Braun of the Milwaukee Brewers admitted to using performance-enhancing drugs in 2011, the year he was National League MVP. He said he took a cream and a lozenge with banned substances while recovering from an injury. Hide Caption 5 of 17 Photos: Athletes and doping in the era of PEDs In 2013, Lance Armstrong admitted to doping throughout his cycling career. The seven-time Tour de France champion confessed to using testosterone, human growth hormone and blood transfusions to enhance his performance. Hide Caption 6 of 17 Photos: Athletes and doping in the era of PEDs Mixed martial arts fighter Alistair Overeem failed a random drug test in 2012 and admitted to injecting himself with a substance that contained testosterone, prescribed for a rib injury. He was suspended for nine months. Hide Caption 7 of 17 Photos: Athletes and doping in the era of PEDs Barry Bonds, baseball's all-time home run leader, was convicted of an obstruction charge in 2011 after he impeded a grand jury investigating the use of performance-enhancing drugs. Bonds had testified that he thought his personal trainer was giving him arthritis balm and flaxseed oil, not steroids or testosterone. Hide Caption 8 of 17 Photos: Athletes and doping in the era of PEDs Houston Texans linebacker Brian Cushing was suspended for four games after testing positive for a drug called human chorionic gonadotropin in 2010. Hide Caption 9 of 17 Photos: Athletes and doping in the era of PEDs Cyclist Floyd Landis admitted in 2010 to using performance-enhancing drugs for most of his career. Landis used the red-blood-cell booster erythropoietin, known as EPO, along with testosterone, human growth hormone and frequent blood transfusions. He was stripped of his 2006 Tour de France win and suspended from cycling for two years. Hide Caption 10 of 17 Photos: Athletes and doping in the era of PEDs In 2008, Olympic track star Marion Jones was sentenced to six months in prison for lying to federal prosecutors investigating the use of performance-enhancing substances. Hide Caption 11 of 17 Photos: Athletes and doping in the era of PEDs Former New Orleans Saints defensive lineman Charles Grant tested positive for banned substances in 2008 and was suspended for the rest of the season. Hide Caption 12 of 17 Photos: Athletes and doping in the era of PEDs Retired New Orleans Saints running back Deuce McAllister tested positive for a banned diuretic in 2008. Hide Caption 13 of 17 Photos: Athletes and doping in the era of PEDs Shawne Merriman, then of the San Diego Chargers, was suspended for four games after testing positive for steroids in 2006. He retired in 2013 after eight NFL seasons. Hide Caption 14 of 17 Photos: Athletes and doping in the era of PEDs Baseball slugger Mark McGwire evaded questions about steroid use when speaking to Congress in 2005. But in 2010, he admitted that he had used steroids during the 1990s. Hide Caption 15 of 17 Photos: Athletes and doping in the era of PEDs Home-run hitter Sammy Sosa was among the players who tested positive for performance-enhancing drugs in 2003, according to the New York Times. Hide Caption 16 of 17 Photos: Athletes and doping in the era of PEDs Former NFL linebacker Bill Romanowski appeared on "60 Minutes" in 2005 and admitted to using steroids and human growth hormone during his career. Hide Caption 17 of 17

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."

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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.