The NFL has issued a letter cautioning players to be careful about consuming clenbuterol-contaminated meat from China and Mexico, as this substance is banned under the league's performance-enhancing policy.

Images of the notice have been making rounds on social media. The notice warns players to take avoid  "consuming large quantities of meat while visiting those particular countries [as it] may result in a positive test." Several players such as Arizona's Patrick Peterson and Baltimore's Steve Smith helped spread the message.

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"There is some evidence that some meat produced in China and Mexico may be contaminated with clenbuterol, an anabolic agent which is banned by the NFL Policy on Performance-Enhancing Substances," the letter said. "Consuming large quantities of meat while visiting those particular countries may result in a positive test for clenbuterol in violation of the Policy."

"Please take caution if you decide to consume meat, and understand that you do so at your own risk," the notice reads,

Duane Brown from Houston, Texas, learned his lesson the difficult way. Following a trip to Mexico last year, he tested positive for clenbuterol. He was subjected to fines and suspensions. But after investigations for months and showing proofs such as receipts of what and where he ate, his 10-game suspension was lifted in April.

Brown is not the first player to be suspended after consuming contaminated meat from Mexico. In 2011, five players from a soccer team from Mexico all tested positive for clenbuterol. It was later traced to meat they had consumed.

In that same year, during the Under-17 World Cup in Mexico, FIFA discovered that most of the players tested positive for the substance.

Clenbuterol is a fat-burning drug injected into cows to make them meatier. Although it has been officially banned in Mexico, a number of ranchers are still using it.

TagsNFL, clenbuterol, Patrick Peterson, Steve Smith, athletes, Duane Brown