The five-time grand slam champion yesterday admitted failing a doping test at the Australian Open in January for meldonium, which became a banned substance under the WADA code this year.

The former No.1 said she had taken meldonium, a heart medicine which improves blood flow and is little-known in the US, for a decade following various health problems including regular sicknesses, early signs of diabetes and “irregular” results from echocardiography exams.

“I was first given the substance back in 2006. I had several health issues going on at the time,” she said. Sharapova didn’t specify whether she had used it constantly since then.

Meldonium was banned because it aids oxygen uptake and endurance, and several athletes in various international sports have already been caught using it since it was banned on January 1.

Sharapova at the Australian Open in January. Photo: Aaron Favila, AP.

Latvian company Grindeks, which manufactures meldonium, said that four to six weeks was a common course.

“Depending on the patient’s health condition, treatment course of meldonium preparations may vary from four to six weeks. Treatment course can be repeated twice or thrice a year,” the company said in an emailed statement.

While Grindeks has previously stated that the drug can provide an “improvement of work capacity of healthy people at physical and mental overloads and during rehabilitation period,” the company said it believed the substance would not enhance athletes’ performance in competition and might even do the opposite.

“It would be reasonable to recommend them to use meldonium as a cell protector to avoid heart failure or muscle damage in case of unwanted overload,” the company said.

Grindeks did not comment when asked whether someone with the symptoms Sharapova described would be a suitable patient for meldonium. The company said it was designed for patients with chronic heart and circulation conditions, those recovering from illness or injury and people suffering with “reduced working capacity, physical and psycho-emotional overload.”

Following Sharapova’s drug test failure, Russian Sports Minister Vitaly Mutko said he expected more Russian athletes to test positive for meldonium.

Sponsors are rapidly walking away from the marquee players, with Swiss watchmaker TAG Heuer cutting ties – the latest to do so after Nike and Porsche also distanced themselves from the world’s highest-paid female athlete.

The swift response signals a change in attitude among high-profile corporate backers following a series of doping and corruption scandals in world sports.

“We’re now entering a zero tolerance era for sponsors,” said Rupert Pratt, co-founder of sports sponsorship agency Generate.

“It is now seen as not acceptable to ‘stand by your man’ because of the amount of scrutiny corporates are now under.”

Loss of sponsor income would be costly for Sharapova, a five-time Grand Slam winner who earned $US29.7 million ($A39.74 million) last year, according to Forbes magazine, most of it from endorsements, appearances and royalties rather than victories on court.

Fellow athletes had mixed reactions to Sharapova’s announcement that she had tested positive for meldonium, a drug she said she had been taking for a decade to treat diabetes and low magnesium.

The substance, recently banned by the World Anti-Doping Agency (WADA), boosts blood flow and can enhance athletic performance. Sharapova, who lives in the United States, is at least the seventh athlete in a month to test positive for meldonium.

“She’s ready to take full responsibility and I think that showed a lot of courage and a lot of heart,” Serena Williams, the top-ranked player in women’s tennis, told reporters at a briefing ahead of a game in New York.

Others were not so sympathetic.

“I’m extremely angry and disappointed. I had to lose my career and never opted to cheat no matter what,” tweeted former world No. 1 Jennifer Capriati, in a long series of posts attacking Sharapova.

Aries Merritt, a US hurdler, said there was no excuse for Sharapova to be unaware that WADA added meldonium to its latest list of banned drugs effective January 1, which it circulated to competitors.

Sharapova said she had not read an email informing her that meldonium was now banned for use in sport.

She will be provisionally suspended from playing tennis from March 12 and could be prevented from competing for Russia at the Rio de Janeiro Olympics this year.

-AP, Reuters