A new policy will keep certain antibiotics from being used to promote growth in food animals.

A new rule that aims to safeguard essential antibiotics for humans by limiting their use in food animals is now fully in effect.

Under the Food and Drug Administration policy, antibiotics that have been designated “medically important” — in other words, they’re needed to treat people — cannot legally be given to healthy animals to speed their growth. The policy, three years in the making, required producers of agricultural antibiotics to change labeling on the drugs to make clear they should not be used for so-called growth promotion. All manufacturers agreed to abide by the new rule.