The World Anti-Doping Agency (WADA) is reportedly allowing athletes a trace amount of the banned drug meldonium which could clear athletes who have tested positive for it.
The announcement came as it remains unclear how long the substance remains in the system. WADA announced last autumn that meldonium would be placed on the banned-substance list as of January 1.
A total of 172 athletes, led by Russian tennis star Maria Sharapova, have since tested positive for meldonium and are suspended.
WADA sent a statement to national anti-doping agencies providing "clarification" on its inclusion on the prohibited list, notice of existing and continuing studies on its excretion from the body and the results management and adjudication process.
It has led to reports that suspended athletes who took in the substance before January 1 could be cleared and for instance compete at the Rio de Janeiro Olympics in August.
It is believed this could apply to athletes with a concentration of under 1 microgram of meldonium per millilitre in test samples collected before March 1.
Up for further review to determine the time of the intake are cases of athletes with concentrations of up to 15 micrograms in tests before March 1, and with less than 1 microgram in tests after March 1.
"There is no doubt as to the status of meldonium as a prohibited substance," WADA president Sir Craig Reedie said.
"There is equally no doubt that the principle of strict liability under the code; as well as, the well established process for results management and adjudication prevail."
He said that since meldonium was added to the prohibited list on January 1 there have been 172 positive examples, but WADA has been asked for further clarification and guidance from its stakeholders,
"WADA recognizes this need - that meldonium is a particular substance, which has created an unprecedented situation and therefore warranted additional guidance for the anti-doping community," he said.
The WADA statement added: "There is currently limited data available on excretion studies relating to meldonium; and, as such, several studies are currently being conducted involving WADA accredited laboratories, which WADA will share when available."
On Friday, Russia's sport minister Vitaly Mutko threatened to take legal action against the WADA if it did not provide a grace period for Russian athletes who used meldonium before it was banned.
The governing biathlon body IBU last week suspended proceedings against two Ukrainian athletes until the results of studies on how long meldonium remains in the system.
Mutko on Wednesday welcomed the WADA release but told Rossiya 24 TV it is "premature" to speak of an amnesty.
"This is only a recommendation from WADA, which held a research and found out that one of the peculiarities regarding meldonium is that it takes up to two months for the substance to leave the body system.
"The drug, however, could remain in the body system much longer depending on the consumed dose. It also depends on the schedule of the drug’s consumption and physical peculiarities of each individual body. Each case is individual and must be treated separately."
The drug was developed by Soviet/Latvian Ivars Calvins to aid blood circulation, and is mainly produced in Latvia.
German anti-doping expert Fritz Soergel meanwhile told dpa scientific research has shown that the substance does not stay in the system very long.
Soergel questioned why meldonium was added to the banned list in the first place.
"There is not much in scientific literature which qualifies this substance as a doping substance. For me it is a nutrition supplement, at most," Soergel said.
The International Tennis Federation (ITF) meanwhile would make would make no comment Wednesday on the Sharapova case, citing confidentiality.
Sharapova is currently suspended and is due a hearing with no date yet made public. It is believed it will be held before the end of this month.
ITF president Dave Haggary criticiized WADA, saying: "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.
"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."