Clenbuterol, a growth enhancer used for cattle, has been banned in Mexico for fourteen years, but some Mexican cattle producers haven't been able to discontinue its use, especially near Mexico City. The ranchers use clenbuterol because it helps their cattle bulk up and makes ranchers more money, Tim Johnson reported for McClatchy DC.
Clenbuterol can cause heart palpitations, tremors, dizziness, nausea and anxiety in people who ingest it or the meat tainted with it. The symptoms usually only last two to six days.
Meat tested in the Mexican state of Guanajuato showed that 30% of the 175 samples testd showed clenbuterol. However, most of the tainted meat is found only in central Mexican states. Outside of those states, it is unlikely that people will feel sick from eating beef.
Read more here.