There were no immediate reports of injuries or illness.
Perrigo Co. said it discovered the metal bits during quality-control checks.
The recall affects bottles containing various amounts of 500-milligram caplets.
Perrigo bills itself as the world's largest manufacturer of store-brand nonprescription drugs. The company did not disclose the chains for which it manufactures the store-brand acetaminophen.
However, Wal-Mart Stores Inc., CVS Corp., Walgreen Co. and Costco Wholesale Corp. are among the companies it supplies with health care products, according to company Securities and Exchange Commission filings.
It was not immediately known if Perrigo has international customers.
The name-brand version of acetaminophen is Tylenol, and generic acetaminophen is often found on store shelves right next to Tylenol products. However, the recall does not affect Tylenol, and it should not cause a shortage of acetaminophen, the FDA said.
Perrigo said the pills contained raw material purchased from a third-party supplier and affected 383 batches.
The voluntary recall is considered a Class II recall since it covers products that might cause a temporary health problem or pose only a slight threat of a serious nature, according to the FDA.
Consumers who swallow any of the contaminated pills could suffer minor stomach discomfort or possible cuts to the mouth and throat, the FDA said, adding that the risk of serious injury was remote. Anyone who suspects they have been injured should contact their doctor, the agency said.
The FDA said Perrigo began investigating after realizing the equipment it uses to make pills was wearing down prematurely.