Athletes who tested positive for meldonium before March 1 could have their bans overturned less than four months before the Rio Olympics after WADA said it was unable to establish how quickly the drug, outlawed since January 1, cleared the system.

The World Anti-Doping Agency’s notice to national anti-doping bodies is expected to have a major impact on many of the 172 athletes who have tested positive for the performance-boosting drug since January.

They include five-times grand-slam tennis champion Maria Sharapova, who was among 40 Russian athletes to test positive for the drug after it was added to WADA’s list of banned substances in January.

Sharapova’s lawyer John Haggerty accused WADA on Wednesday of “poorly” handling the issue and said they were now “trying to make up for it”.

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.

The anti-doping body’s notice also gave hope to athletes who have tested positive for the drug since March 1, depending on studies being carried out to determine how long it stays in the body.

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

“The fact that WADA felt compelled to issue this unusual statement now is proof of how poorly they handled issues relating to meldonium in 2015,” Haggerty said in a statement.

“Given the fact that scores of athletes have tested positive for taking what previously was a legal product, it’s clear WADA did not handle this properly last year and they’re trying to make up for it now.”

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

“The situation with Sharapova could be resolved after April 21 when we meet with the head of the international federation. It is too early to talk about Sharapova competing at the Olympic Games,” Russia’s TASS news agency quoted Tarpishchev as saying.

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.

“WADA has demonstrated impartiality and being objective in the fight against doping.”