When you’re sitting in the dentist’s chair waiting to have a cavity filled, you don’t see the package the amalgam filling comes in. But if you did your eye might well be drawn to a couple of “contraindications,” med-speak for “situations in which the dentist should not use this product.” In addition to ho-hum warnings about not using the amalgam, which contains about as much mercury as a thermometer, in patients known to be allergic to amalgam (duh), the manufacturers say it should not be used in children age 6 and under, or in pregnant women.

The reason, of course, is that mercury is a known neurotoxin, especially dangerous to developing brains. For decades anti-mercury activists have pushed the industry to develop substitutes (so-called composites, or resins, are now available), and even to persuade people to have their fillings ripped out, but have made very little headway.

Now they have won a big one. In a legal settlement signed last week, the U.S. Food and Drug Administration has changed the information on its Web site about amalgam fillings to say that they “may have neurotoxic effects on the nervous systems of developing children and fetus,” and that pregnant women “should not avoid seeking dental care, but should discuss options with their health practitioner.” Previously, there was no such warning. The FDA also agreed to decide by next year whether mercury fillings need more regulation.

The FDA had been dragging its feet for so long on mercury in dental fillings that even the judge, Ellen Segal Huvelle of U.S. District Court for the District of Columbia, castigated the agency: “This is your classic failure to act,” she said. (The case is Moms Against Mercury et al. v. Von Eschenbach, Commissioner, et al. The “et al” plaintiffs are other consumer and environmental groups, as well as state officials.)

In a press conference yesterday, the anti-mercury groups claimed victory: “Gone are all of FDA’s claims that no science exists that amalgam is unsafe, or that other countries have acted for environmental reasons only, or that the 2006 FDA advisory panel affirmed amalgam’s safety, all of which are untrue,” said plaintiff Michael Bender of the Mercury Policy Project.

The foot-dragging that the judge called the agency on has been particularly noticeable since 2006, when members of an FDA advisory panel recommended [http://www.fda.gov/cdrh/meetings/090606-summary.html ] that the agency consider informed consent for patients receiving amalgam fillings, as well as possible labeling changes restricting their use in pregnant woman and children. None of that happened, although a number of other countries are much more cautious when it comes to amalgam fillings, including Canada, France and Sweden.

So what does the science say about any risks posed by the mercury in amalgam fillings, which make up about 30 percent of the 150 million fillings used each year in the U.S.? Don’t bother with the many old studies concluding that they pose no risk, for until 2006 there has not been a single one following the gold-standard for this kind of research, namely, randomized controlled trials following children with and without mercury fillings for at least five years. The first (and so far only) two [click here and here ] were published in the Journal of the American Medical Association in April 2006. The conclusion: after following just over 1,000 kids for five years in one study and seven years in the other, the scientists found no evidence of harmful effects. As one team put it: “no statistically significant differences were found between children in the amalgam and composite groups” in IQ, in memory abilities, in visuomotor skills. In all, “there were no statistically significant differences in adverse neuropsychological . . . effects observed over the 5-year period in children whose caries were restored using dental amalgam or composite materials . . . [a]though it is possible that very small IQ effects cannot be ruled out.”

Here’s where it gets interesting. The JAMA editors commissioned an editorial on the two papers. It was written by Herbert Needleman, the scientist who did more than any other to document the toxicity of lead to developing brains. He didn't exactly find the studies to be the last word on the subject, writing, “there are, as the authors clearly delineate, limits to the inferences that can be drawn from these data. It is predictable that some outside interests will expand the modest conclusions of these studies to assert that use of mercury amalgam in dentistry is risk free.” (Indeed, the American Dental Association continues to do exactly that.) “This conclusion,” Needleman said, “would be unfortunate and unscientific.”

Why? Even the impressive 5- and 7-year periods that the children were followed and tested may not be enough, for early exposure to toxic compounds can have effects even later in life. Said Needleman, “The hints that mercury has an effect on the aging brain emphasize” that concern. Also, even with 1,000 children the studies cannot rule out the possibility that mercury fillings cause a loss of several IQ points. Since 50 million children are walking around with amalgam fillings, “if mercury caused subtle effects in 1% of those exposed, up to 500 000 children could be affected,” Needleman pointed out.

One wild card here is that, as scientists are finding with so many other toxic compounds, who’s harmed depends on genes. Millions of baby-boomers with a mouth full of fillings are probably saying right about now that their brains work just fine, thank you, despite decades of living with mercury in their teeth and therefore mercury vapors wafting into the blood and brain. But “sensitivity to mercury toxicity may have a genetic basis,” Needleman wrote: variations in a gene called coproporphyrinogen oxidase (CPOX4) “altered the impact of mercury on cognitive and mood scores. Approximately 25% of the US population” has the sensitive-to-mercury gene.

What to do? No one with any sense recommends having your existing mercury fillings ripped out. That’s not only costly and possibly painful, but could expose you to a big pulse of mercury. Still, surely it is reasonable to stop pretending that putting neurotoxins in our mouths-where they stay for decades, with tiny puffs of mercury being released every time we chew-is a good idea. After all, there was also a time when "experts" pooh-poohed the suspicion that lead was bad for children's brains, too.