Maryland has become the first state to ban an arsenic additive in chicken feed, according to reports.

A law banning the additive took effect on January 1. In 2012, the state approved 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 2012 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.

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Opponents said the measure wasn't needed because the company voluntarily suspended sales of the product.