By Toni Clarke

(Reuters) - The U.S. Food and Drug Administration on Friday moved to revoke approval of a drug used to treat certain diseases in pigs because it could leave a cancerous residue that may affect human health.

The drug, carbadox, is made by Teaneck, New Jersey-based Phibro Animal Health and is used to control swine dysentery and bacterial enteritis, the agency said. The drug, which was approved in 1972, has also been used to promote weight gain in pigs.

"Potential cancer risks are based on an assumed lifetime of consuming pork liver or other pork products containing carbadox residues," the agency said in a statement, adding that it is not recommending that people change their food choices while it works to remove the drug from the market.

"Pork is a good source of protein," the agency said. "However, protein can also be found in other meat, poultry, seafood, beans and peas, eggs, processed soy products, nuts and seeds."

Pork liver is used to make liverwurst, hot dogs, lunch meat and some types of sausage, the agency said. The product is sold under the brand name Mecadox.

Phibro officials insist the product is safe.

"Mecadox has been approved and sold in the United States for more than 40 years and is a widely-used treatment for controlling bacterial diseases," Phibro said in a statement.

Dave Warner, a spokesman for the Pork Producers Council, which represents the pork industry, said it is reviewing the implications of the FDA decision but will let the Animal Health Institute, which represents animal health drug companies, “take the lead.”

A spokesman for the Animal Health Institute was not immediately available for comment.

The FDA said it asked Phibro for additional information about the safety of carbadox, but the company has not submitted any proof that there is a safe way to use it.

Phibro said it has cooperated fully with the FDA's inquires, "including undertaking a comprehensive, rigorous new studies using the latest and most sensitive technology available." To date, the company added, that evidence has been positive.

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It said its studies are due to be completed in the next 90 days, and "we expect that the remaining evidence will support the continued safe use of Mecadox."

Phibro has 30 days to request a hearing on the matter and said it will do so. The FDA's commissioner, Dr. Robert Califf, will then decide whether a hearing is justified.

Shares of Phibro closed down 2.3 percent at $26.89 on the Nasdaq on Friday.

Tom Burkgren, executive director of the American Association of Swine Veterinarians, said it is unclear whether other products will be as effective as carbodox in treating gut problems in pigs, but said the industry will adjust if carbadox is withdrawn.

"We'll accept it and we'll move on," he said.

The FDA's actions follow a preliminary risk assessment conducted from 2012 to 2014 which found that the lifetime cancer risk from consuming pork liver containing carbadox residue is higher than allowed under the FDA's framework for regulating carcinogenic animal drugs.

Pork producers have become more interested in carbadox recently because it does not require a veterinarian's prescription, according to John Goihl, president of Agri-Nutrition Services Inc, a Minnesota-based firm that provides feed formulations and consulting services to manufacturers and livestock producers.

In 2012 the FDA issued a rule requiring farmers to obtain a prescription before using antibiotics in an attempt to curb the practice of feeding antibiotics to livestock just to make them grow fatter. Drugs that do not have a human equivalent are exempt.

Three antibiotics made by Phibro contain carbadox: Mecadox Premix 10, Banminth/Mecadox; and Mecadox/Terramycin, the FDA said.

(Reporting by Toni Clarke in Washington; Additional reporting by Tom Polansek in Chicago; Editing by Leslie Adler)