WASHINGTON (Reuters) - U.S. health regulators have warned Pfizer Inc over a series of failures that led to the overdosing of at least 13 children in a clinical trial of its antipsychotic drug Geodon, according to a letter made public on Tuesday.

The FDA, in an April 9 warning letter to the world’s largest drugmaker, said Pfizer “failed to ensure proper monitoring” of the trial in which several children given overdoses experienced tremors, restless legs and other complications.

The company is seeking to market the drug to children with bipolar disorder and has received cautious support from a U.S. advisory panel. While the study in question ended in 2007, the FDA is concerned Pfizer has not done enough to ensure the problem does not happen again.

Its concerns come amid continuous unease over the use of powerful drugs to treat depression, schizophrenia, bipolar disorder and other mental illness in youth. Many experts are concerned that such medications -- widely used to treat adults -- may not work the same in children and teenagers whose brains are still developing.

Geodon, first approved in 2001 for adults with schizophrenia, is already cleared for adults with bipolar disorder. Companies which study approved drugs in children can not only seek approval of pediatric use of their therapies, but also an added 6 months of patent protection under U.S. law.

In its letter, the FDA said the company did not properly monitor the study and “as a result of inadequate monitoring, widespread overdosing of study subjects at multiple study sites was neither detected nor corrected in a timely manner.”

Pfizer said it recognizes the issue’s seriousness and is committed to addressing the concerns, adding it reported many items cited in the letter as long as four years ago.

Since then it “has instituted several new measures designed to improve monitoring and execution of clinical trials, including our oversight of clinical investigators,” it said.

None of the children in question suffered long-term side effects or injuries because of the overdosing, Pfizer added, and just two of the patients were removed from the study.

The FDA said the letter follows repeated violations that the agency had notified Pfizer about in 2005. Seven children were found to have been overdosed, and an internal Pfizer document pointed to six others, it said.

The letter released on Tuesday comes after a 2009 agency inspection. A June 2009 response from Pfizer acknowledged the problems, but did not include a detailed list of the drugmaker's plans to prevent future problems, the FDA said in its letter, posted online at link.reuters.com/waf68j.

At the same time, FDA was also seeking advice on the use of such drugs in younger patients.

Last June, an FDA panel of outside advisers cautiously backed Geodon along with two other similar drugs -- AstraZeneca Plc’s Seroquel and Eli Lilly and Co’s Zyprexa -- saying they appeared safe and effective in children, although concerns remained over their long-term impact.

Pfizer shares fell 3 cents to close at $16.76 on the New York Stock Exchange, roughly in line with the overall drug sector.