WASHINGTON - Prodded by consumer activists, the Food and Drug Administration is reviewing the scientific evidence underlying its pronouncement less than 18 months ago that dental fillings containing mercury do not cause harm.

An advisory panel of outside experts will meet this week to reexamine the basis of the FDA's conclusions.

It is the latest chapter of a lengthy battle with groups that believe the agency is understating possible links between the mercury in dental amalgam and neurological and other health problems.

The review is "not being taken because of new data," and "at this time, the FDA is not modifying its existing guidance" that mercury fillings are safe, Nancy Stade, deputy director for policy in the FDA's medical device center, said in a briefing Thursday.

But questions about the rigor of the FDA's science, contained in four citizens' petitions to the agency, justify a second look at the evidence, Stade said.

Restrictions on amalgam, which has been used in hundreds of millions of people over the last 150 years, are opposed by the American Dental Association, which says decisions about use of the substance should remain between patients and their dentists.

The FDA's current position on mercury fillings followed a lawsuit by anti-amalgam groups that sought to have the substance placed in an FDA classification scheme for medical devices as a first step in restricting and eventually banning its use.

In settling the suit, the FDA in July 2009 classified amalgam as a moderate-risk item that released levels of mercury "not high enough to cause harm in patients."

The FDA chose not to require warnings about possible harm to young children and women of childbearing age, said Charles Brown, an attorney for Consumers for Dental Choice, one of the groups that sued the FDA and petitioned the agency to look again at the issue.

Documents released ahead of the advisory meetings Tuesday and Wednesday indicate that "they're certainly digging into the science," Brown said. "But I just don't know if they can stand up and say to the FDA staff that you've been wrong for the last 25 years."

The advisory committee will not vote on specific recommendations, but will discuss a series of technical questions about how exposure to mercury is measured, whether safe levels of exposure have been set correctly, and the reliability of studies of the effects of mercury on humans.

If the FDA determines that amalgam fillings pose a risk, millions of people would have to decide whether to have them removed and, if so, how to pay for that.

Amalgam releases mercury vapor, high levels of which are associated with brain and kidney disorders.