A performance-enhancing substance sold on the black market which contains "serious toxicities" could seriously damage athletes' health, the World Anti-Doping Agency (WADA) said on Thursday.
WADA said the once developmental drug, identified as GW501516, had been bought and used by some athletes despite being withdrawn from research and terminated when "serious toxicities were discovered in pre-clinical studies.
"The side effect of this chemical compound is so serious that WADA is taking the rare step of warning "cheats" to ensure that there is complete awareness of the possible health risks to athletes who succumb to the temptation of using GW501516 for performance enhancement," WADA added in a statement.
It said the drug did not have, and would not get, clinical approval.
"However, GW501516 has been available for some months on the black market, through the Internet and elsewhere. Anti-Doping authorities have already seen its use by athletes, as there are a number of positive cases."
WADA did not specify which athletes had tested positive for the substance but urged federations to notify athletes "as soon as possible."