The warning letter, posted on the F.D.A. Web site Tuesday, cited Perrigo for shipping defective drug products from its Allegan factory. The plant had identified certain ibuprofen tablets as being contaminated with metal shavings but had shipped a portion of the pills anyway, the letter said, before later recalling the entire lot. Mr. Shannon of Perrigo said that the company had recalled the ibuprofen tablets cited in the letter from a retail warehouse before the pills reached retail store shelves. In an earlier incident in 2006, Perrigo recalled about 11 million bottles of acetaminophen because it found metal particles, ranging from a speck to 8-millimeter pieces of wire, in some caplets.