MLB players have been issued a warning over the use of deer-antler spray, a substance administered under the tongue that includes a banned chemical known for its muscle-building and fat-cutting effects, SI.com has reported.

Players had felt free to use the spray at nearly no risk until the warning was sent last week by the league, the report said.

In its warning, issued in reaction to reports from the drug-testing industry, MLB requested players not use the spray because it contained "potentially contaminated nutritional supplements" and had been added to the league's cautionary list of products.

The warning was not issued because the spray includes the banned chemical, SI.com reported.

IGF-1, or insulin-like growth factor, can't be detected in the urine tests used by baseball, and the players' association has not come to an agreement with MLB on blood testing.

According to the report, scientists discovered IGF-1 in the velvet of immature deer antlers and players have been using it as an alternative to steroids.

The chemical is considered a performance-enhancer and its use is prohibited by baseball and the World Anti-Doping Agency, among other governing bodies. IGF-1 is said to mediate the level of human growth hormone in the body, SI.com reported.

MLB said in its warning that the spray can cause players to test positive for the banned steroid methyltestosterone, though it is not listed as an ingredient, the report said.

Among the benefits highlighted by manufacturers of the spray are "anabolic or growth stimulation," "athletic performance" and "muscular strength and endurance."