Pfizer received a warning letter from the U.S. Food and Drug Administration (FDA) last week, citing the pharmaceutical giant for inadequate clinical trial monitoring in the study of a pediatric drug—violations that led to the overdosing of at least 26 children.

The drug, unnamed in the FDA’s letter, is reported to be Geodon, which is used to treat schizophrenia and bipolar disorder. Pfizer is seeking FDA approval for the drug to be used to treat children between the ages of 10 and 17 years.

Last week’s warning was the latest in a string of FDA-Pfizer letters regarding violations in the Geodon study.

According to the FDA’s letter, Pfizer failed to ensure proper monitoring at several clinical trial sites between 2005 and 2009, which resulted in overdosing of participants for several days—one for as long as 22 days.

Pfizer received its first letter related to the pediatric study in 2005, when an FDA investigation revealed “widespread overdosing of study subjects at multiple study sites.” Despite efforts by Pfizer to improve the monitoring process, additional overdoses and monitoring violations were uncovered in 2006 and 2007. These problems were identified and reported to the FDA by Pfizer itself, but the FDA noted that 2006 discovery was made by a Pfizer data management unit, not a study monitor.

Pfizer took steps to remedy the problem by re-training staff and adding monitors, but a July 2009 FDA inspection prompted the latest warning letter when investigators discovered that Pfizer was still not following its own clinical trial safety guidelines. The drugmaker responded with a letter to the FDA, ensuring that efforts would be made to protect participants in the future, but, in its latest warning, the FDA said Pfizer needs to provide a more specific outline for ensuring proper monitoring.

In a printed response on the company’s web site, Pfizer was quick to point out that many of the violations had been discovered by the company itself.

“Many of the items cited by the FDA were first uncovered and reported to the FDA by Pfizer as far back as four years ago as part of our ongoing clinical trial monitoring and quality assurance processes. Since that time, Pfizer has instituted several new measures designed to improve monitoring and execution of clinical trials, including our oversight of clinical investigators,” the statement said.

Pfizer has 15 days to respond to the FDA’s letter, which was dated April 9. Pfizer said that it had communicated with the FDA and would spend the next two weeks creating a plan to ensure that similar errors do not occur in the future.