MONTREAL (AP) - The World Anti-Doping Agency is warning athletes against using a black-market drug that causes serious health risks.

WADA sent a letter to anti-doping organizations Thursday urging them to notify athletes of the dangers of the substance known as “GW501516.”

The letter says “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.”

WADA says the product, which reportedly can increase an athlete’s endurance, was withdrawn from research by a pharmaceutical company after toxic properties were found in pre-clinical studies.

WADA says the drug has been available for months on the black market and resulted in a “number of positive cases” among athletes.

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