The NFL has warned players that some meat produced in China and Mexico may be contaminated with clenbuterol, a banned steroid.

In a memo sent out by the league office, the NFL warned players that consuming large quantities of meat while visiting those countries may result in a positive test for clenbuterol. The memo asked the players to "please take caution if you decide to consume meat, and understand that you do so at your own risk."

The league also reminded players that they are responsible for any substances found in their bodies.

Clenbuterol has been banned in Mexico as a growth enhancer for cattle but is suspected to still be in use there.

The Oakland Raiders are scheduled to face the Houston Texans on Monday Night Football on Nov. 21 in Mexico City as part of the league's international series.