The letter goes on to cite over 10 "musty-moldy odor" complaints about Rolaids and over 39 similar complaints about Tylenol Extra Strength, "including three adverse event reports."
The letter is dated Jan. 8, 2010, but was posted to the FDA's Web site Jan. 13.
"Certainly, the FDA report raises serious questions about the manufacturer's response," said Robert Field, professor of health management and policy at the Drexel University School of Public Health. "The report has found that the investigation was limited...that the procedures for quality control were not in writing...and various other lapses that were fairly significant."
The FDA report further criticized McNeil for inadequate responses to complaints, noting that they first received heightened complaints about the musty odor in 2008 and testing confirmed its presence in September of that year.
"Your quality unit failed to conduct additional testing to evaluate the possibility of chemical contamination or other change or deterioration in the distributed drug product," the FDA letter says.
Karen Hirshfield, acting branch chief of the FDA Office of Compliance's Recalls and Shortages Branch, revisted these concerns during this morning's briefing.
"We would have expected McNeil to expand their investigation when they first learned of consumer complaints on this issue," she said. "They became aware of a problem in September 2008, and their investigation and report to [the FDA] didn't occur until about a year later. We would have expected action to occur sooner than that."
Meanwhile, current problems with Rolaids and Tylenol Extra Strength are attributed to another testing failure.
"Investigations of an unexplained discrepancy did not extend to other batches of the same drug product and other drug products that may have been associated with the specific failure or discrepancy," the report said.
In response to an inquiry from ABC News correspondent David Kerley, the company responded on Thursday:
"McNeil Consumer Healthcare has received an FDA form 483 dated Jan. 8, 2010, and is actively working with the FDA to address their concerns."
One positive for consumers is that, as ABC News Senior Medical Editor Dr. Richard Besser noted in December, after the first recall was announced, the effects of the tainted pills were not life threatening.
"They're not that severe, we're talking about vomiting, diarrhea," said Besser. However, he advised, "if you have this product, you should get rid of them, safely dispose of them in your trash. You don't want them around."
Field agreed, following news of the initial recall.
"It does not look like there were serious health effects, if any," he said, "but why take a chance? These products are supposed to improve your health, not hurt it."
ABC News correspondent David Kerley contributed reporting from Washington, D.C.