The FDA was responding to critics who say the agency is toothless when it comes to tracking the safety of drugs already on the market, when industry funds that supported pre-approval reviews tend to dry up.

"We think we've really balanced this," Dr. Janet Woodcock, director of the agency's Center for Drug Evaluation and Research, told reporters attending the Association of Healthcare Journalist meeting in Atlanta.

In a report released on Saturday, the FDA says it has required companies to do 385 post-market studies since 2008, and to change the label based on new safety information 65 times.

Before 2008 the FDA could not compel label changes or require additional safety trials and had to rely on voluntary action from drugmakers, she said.

The FDA's focus on post-market drug safety comes after the agency was sharply criticized as being slow to respond to drug side effects, notably Vioxx, a painkiller that Merck & Co Inc pulled five years after approval because of a link to heart attacks and strokes.

A 2006 report from the Institute of Medicine, an independent research body that advises the government on scientific matters, found the FDA needed to do more to police the safety of medicines after they reach the market.

Congress later gave the FDA more power to oversee drugs after approval, such as ordering companies to change their labels or conduct additional safety trials, and fining them if they fail to do so.

SENTINEL

But critics question how often the agency actually follows through on penalties.

In 2008 the FDA also launched Sentinel, a computer tracking system designed to help identify problems with drugs and medical devices already on the market by searching various databases for possible side effects - rather than just relying on voluntary reports from companies and patients.

The FDA said it used the system to see if there were negative drug interactions for Chantix, the smoking cessation drug manufactured by Pfizer Inc, but did not find any.

Last November U.S. researchers said Chantix carried too many risks and should only be tried when other treatments fail.

The findings, which Pfizer said were flawed, contradicted two studies released in October by the FDA that showed Chantix (sold as Champix outside the United States) did not increase the risk of being hospitalized for psychiatric problems such as depression.

The agency at the time acknowledged those studies were flawed because they were too small to identify rare events and captured only cases severe enough to land people in the hospital.

Woodcock could not immediately provide examples of where Sentinel alerted the agency to safety information they had not been aware of before.

(Additional reporting by Anna Yukhananov in Washington; Editing by Xavier Briand)

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