Maryland has become the first state to ban an arsenic additive in chicken feed.
A law banning the additive is among new state laws taking effect on Tuesday.
Last year, state lawmakers passed a bill banning the use of Roxarsone, a chemical used to help the birds grow and fight parasites.
Pfizer Inc., which distributes Roxarsone, stopped selling the chemical in July after a U.S. Food and Drug Administration study found higher levels of inorganic arsenic in chicken treated with the additive than those that were not. Arsenic is a known carcinogen.
Opponents said the measure wasn't needed because the company voluntarily suspended sales of the product.