Athletes who tested positive for meldonium before March 1 this year could have their bans overturned after the World Anti-Doping Agency (WADA) announced it was unable to establish how quickly the drug, outlawed since January 1, cleared the system.

WADA’s announcement could allow five-times grand-slam tennis champion Maria Sharapova, who failed a test for meldonium in January, and other banned athletes to return to competition, Russian officials said yesterday.

Sharapova, the world’s highest-earning sportswoman, was among 40 Russians who failed dope tests for meldonium after it was added to WADA’s list of banned substances in January.

WADA said there was “currently a lack of clear scientific information on excretion times.

As a result it is difficult to know whether an athlete may have taken the substance before or after January 1, when it became illegal.

“In these circumstances, WADA considers that there may be grounds for no fault or negligence on the part of the athlete,” it said in a statement sent to anti-doping agencies and sports federations, adding that the presence of less than one microgram of meldonium in the samples was acceptable.

Sharapova, who said she had been taking meldonium for more than a decade because of health problems, was banned by the International Tennis Federation (ITF) on March 12 after announcing she had failed a test at the Australian Open in Melbourne.

Russian Tennis Federation president Shamil Tarpishchev said Sharapova’s suspension could be addressed in a meeting with ITF head David Haggerty later this month.

Meanwhile, Russian Sports Minister Vitaly Mutko welcomed WADA’s decision.

“The Russian Sports Ministry supports and welcomes the decision made by WADA because it has shown a willingness to understand the situation, rather than stick to the rulebook,” Mutko said in a statement.

Meldonium, manufactured for people suffering from heart problems, can also increase blood flow and improve exercise capacity.