WASHINGTON – Federal health scientists have panned a GlaxoSmithKline study that the company used to defend the safety of its embattled diabetes drug Avandia, a once blockbuster-seller that has fallen out of favor because of potential ties to heart attacks.

The Food and Drug Administration posted an exhaustive 700-page review of Avandia on Friday ahead of a meeting next week to decide whether the drug should stay on the market.

The FDA finds itself in a difficult position that’s all too familiar: reviewing a drug approved a decade ago that now appears tied to deadly side effects.

Experts say the FDA’s predicament is a result of shifting standards for the agency: increased scrutiny on safety and stepped-up pressure from Capitol Hill.

The FDA reviewed dozens of studies of Avandia, including one Glaxo has pointed to as proof of the drug’s safety. But an FDA reviewer said the study was plagued by “serious flaws” and actually supports the case against Avandia.

The drug works by increasing the body’s sensitivity to insulin, a key protein needed for digestion that diabetics don’t adequately produce.

People with diabetes are unable to properly break down carbohydrates, either because their bodies do not produce enough insulin or because of resistance to insulin.

They are at higher risk for heart attacks, kidney problems, blindness and other serious complications.

Avandia was Glaxo’s third best-selling drug in 2006 with U.S. revenue of $2.2 billion, according to health care statistics firm IMS Health. But safety concerns swirling around the drug have pummeled sales over the last three years, with sales falling 75 percent to $520 million last year.

In 2007 an analysis of dozens of studies first linked the drug to heart attacks.

The FDA responded by adding a warning label to the drug later that year.

Glaxo, based in London, has argued for years that Avandia’s safety should be assessed only based on clinical trials, considered the gold standard of medical research.

But the FDA reviewer said Glaxo’s chief trial “was inadequately designed and conducted to provide any reassurance” about the heart safety of Avandia.

The FDA holds a special two-day meeting starting Tuesday to help decide what course of action to take. A panel of outside physicians will consider a range of recommendations including:

• Adding more warning labels.

• Limiting which doctors can prescribe the drug.

• Pulling the drug from the market.

The FDA is not required to follow the advice of its outside panels, though it usually does.

The agency has been down this road before.

In 2007 the FDA assembled the same group of experts to vote on the same drug. The group voted 22-1 in favor of keeping Avandia on the market.

New data on Avandia’s risks and pressure from politicians have prompted the agency to re-examine the drug’s safety.

Despite the reams of information posted online, the FDA’s main problem remains one of too little data.

Avandia, like many other drugs of the 1990s, was approved based on relatively small studies in several thousand patients. While those studies were sufficient to show the drug helped control blood sugar levels — the key measure for diabetes drugs — they were not large enough to detect all of the drug’s potential side effects.