The World Anti-Doping Agency (WADA) has said athletes could escape a ban for taking meldonium because it does not know for sure how long it takes the substance to leave the body.
Meldonium was banned on January 1 and WADA said there have since been 172 test failures, including Russian tennis star Maria Sharapova.
Russia has been particularly badly hit by the meldonium scandal with track and field athletes and swimmers among about 40 Russians caught.
WADA said that for meldonium, which has been used to treat heart disease and diabetes but also enhances athletic performance, "there is currently a lack of clear scientific information on excretion times".
"For this reason, a hearing panel might justifiably find (unless there is specific evidence to the contrary) that an athlete who has established on the balance of probabilities that he or she ingested meldonium before January 1, 2016 could not reasonably have known or suspected that the meldonium would still be present in his or her body on or after 1 January 2016.
"In these circumstances, WADA considers that there may be grounds for no fault or negligence on the part of the athlete."
WADA also said action could be suspended if between one and 15 micrograms of meldonium are detected and the test was taken before March 1. The world agency said this is because studies are needed to determine when a substance was taken.