Officials at the Food and Drug Administration said the amounts found were so low that chickens treated with the drug, called 3-Nitro, do not pose a serious health risk and will continue to be sold. Perdue and organic chicken producers do not use the drug.
“The presence of that carcinogenic residue obviously raises concerns, and it’s completely avoidable,” said Michael R. Taylor, F.D.A.’s deputy commissioner for foods. “It needed to be acted on.”
Pfizer, which makes 3-Nitro, also known as roxarsone, will suspend the drug’s sales in 30 days, giving producers time to find alternatives. The drug, first approved in 1944, kills intestinal parasites, promotes growth and makes meat look pinker. And since 3-Nitro contains organic arsenic, which is far less toxic than its inorganic counterpart, producers assumed that it would have no effect on people who ate the animals.
But there has been growing evidence that organic arsenic can change into its more toxic cousin. So F.D.A. researchers developed a way to measure inorganic arsenic in meat. They got 100 chickens, fed roxarsone to about half of them and measured levels of inorganic arsenic in their livers. Chickens fed roxarsone had consistently higher levels of inorganic arsenic, a known carcinogen.
Scott Brown, a veterinarian at Pfizer Animal Health, said that Pfizer’s own studies of 3-Nitro had all found it safe, but that the F.D.A.’s trial led the company to suspend sales of the drug while Pfizer did a “full scientific assessment” of the drug. That assessment will also include another smaller-selling arsenic-based feed medicine called Histostat, or nitarsone, that was not studied by the F.D.A. and whose sales have not been suspended.
Consumer advocates cheered the move.
“Inorganic arsenic is cancer-causing, and action on this drug is long overdue,” said Dr. Michael K. Hansen, a senior scientist with Consumers Union.
Each year, the United States produces $45 billion worth of broiler chickens, the kind used for meat — about 37 billion pounds in 2010, according to the Agriculture Department. That equals about 80 pounds a year for each American, according to the National Chicken Council, which represents major companies like Perdue and Tyson. Georgia, Arkansas and Alabama are the biggest producers.
Richard Lobb, a spokesman for the chicken council, said 3-Nitro was used widely “but certainly not universally” by the industry. Mr. Lobb said that questions about roxarsone’s safety had been raised in the past but “we’ve always been able to show that there’s really no reason to be concerned about human health.”
Environmentalists have long been concerned that the waste from chickens treated with roxarsone, when used as fertilizer on crops, causes arsenic to leach into water supplies and estuaries. Even cattle are exposed, since chicken litter is sometimes included in feed.
About 90 percent of roxarsone’s use is in chickens, although the product is also used to a more limited extent in turkeys and swine, said Dr. William Flynn of the F.D.A.’s Center for Veterinary Medicine.
The F.D.A. once routinely conducted its own studies of animal and human drugs, but limited budgets led the agency to eliminate much of its scientific and laboratory capacity over the years. The roxarsone study is a triumph for agency scientists but one unlikely to be repeated very often. The agency asked for $183 million in additional funds for food safety efforts next year, but House Republicans have instead proposed cutting $87 million.
“This is exactly the kind of thing we try to direct research efforts towards,” Dr. Flynn said.
Roxarsone is still approved for use in chickens and swine in Canada, Mexico, Malaysia, Indonesia, the Philippines and Vietnam and in poultry only in Chile, Argentina, Peru, Venezuela, Brazil, Australia, Pakistan and Jordan.
Representative Steve Israel, Democrat of New York, proposed legislation in 2009 banning roxarsone. “I’m relieved the decision was made to make our poultry safer and healthier and hope the F.D.A. will take the final step and ban roxarsone altogether,” Mr. Israel said.
Pfizer has promised not to resume sales of the drug until it has resolved F.D.A. concerns about it.
William Neuman contributed reporting.
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