Until Nancy Yost had her “silver” dental fillings removed, she was convinced that she had multiple sclerosis and would never get better.

Her doctor had diagnosed the disease, which attacks the central nervous system. The San Jose, Calif., woman gradually had lost the ability to walk and speak clearly. She was alternating between a cane and wheelchair when the fillings, which are 50 percent mercury, were taken out.

The next night, Sept. 28, Yost went dancing. She has not used her wheelchair since.

Her dramatic improvement has convinced Yost and her dentist that she was a victim of mercury poisoning.

They are the latest converts in the dental amalgam wars — the fierce debate over the safety of mercury in the mouth that has raged for more than 150 years, when dentists started using mixtures of mercury, silver, tin and copper to fill cavities in teeth.

But where 10 years ago Yost and her dentist would have been branded kooks by dental authorities, new scientific studies and a class-action lawsuit filed against the American Dental Association in September by dozens of its own members have added new fire to the old claim that the mercury in these “silver” or amalgam fillings can be a health risk.

Even the federal Food and Drug Administration, which gave amalgam its seal of approval in 1976 without a scientific review, will consider “possible hazards from dental amalgam” in March.

“This will prove to be the biggest boondoggle medical problem of this century,” said Dr. Joyal Taylor, a San Diego dentist and founder of the 700-member Environmental Dental Association. “How can anyone say that mercury is not safe outside the mouth but is safe inside the mouth? Mercury does not belong in the mouth. Mercury belongs in thermometers.”

The American Dental Association, the 140,000-member association for dentists in the United States, continues to sharply disagree with such critics. It estimates that 100 million Americans have amalgam fillings in their mouths but opposes informing patients that the mercury in their fillings could be toxic, even though the ADA supports strict guidelines for disposal and handling of the material by dentists.

“There is no responsible, conclusive scientific evidence to characterize amalgam as anything but safe and effective,” said Philip Weintraub, manager of media relations for the Chicago-based ADA.

It’s only been in the past decade, however, that the type of research needed to determine whether dental amalgam is safe has been undertaken. Results from those studies have established that mercury from silver fillings accumulates in the body, sometimes passes from the mother to a fetus and sharply reduces kidney function in sheep.

No one disagrees that mercury is one of the most toxic substances known. Leading toxicologists have said there is no safe level of mercury exposure, but research has yet to show a direct link between health problems in people and the amalgam in their mouths.

The debate is over whether the mercury vapor inhaled by people with amalgam fillings, which are only a third silver, accumulates in the body enough to be harmful. Autopsy studies in Sweden, for example, have shown that people with amalgam have two to three times as much mercury in their brains and nine times as much in their kidneys as people without silver fillings. But it did not determine whether those amounts were harmful.

Symptoms reported by people like Nancy Yost include central nervous or immune system disorders, depression, anxiety, headaches, back pain and dizziness. The symptoms mimic such diseases as multiple sclerosis and chronic fatigue syndrome.

The first evidence that mercury could escape from hardened amalgam fillings came in 1979, when University of Iowa researchers found that chewing food and grinding teeth constantly releases small amounts of mercury vapor, which can be inhaled into the lungs.

Preliminary results from a subsequent University of Southern California School of Dentistry study indicated that dental amalgam can decrease the count of infection-fighting white blood cells.

And a recent study out of the University of Calgary Medical School in Alberta found that when 12 amalgam fillings each were placed in the mouths of six sheep, the animals lost half their kidney function within 30 days.

Such studies have prompted five states to consider requiring dentists to get a patient’s consent before inserting amalgam fillings. Sweden is considering a ban.

The ADA, however, remains adamant that “dental amalgam is a safe, effective restorative material,” the only one that many people can afford. The dental trade group conceded a few years ago that fillings release mercury vapor but says the amounts are too insignificant to be harmful.

“Dental amalgam continues to be the material of choice for the clear majority of dentists,” said Weintraub of the ADA.

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Critics say many dentists continue to use amalgam because it is easier to work with than newer materials, such as resin composites or porcelain inlays and crowns. They also stress that many dentists are intimidated by the ADA.

And with reason. It is a violation of ADA policies for a dentist to remove amalgam fillings for any other than cosmetic reasons. Several dentists have had their licenses suspended for doing so.

“Every year, dentists pack another 100 tons of mercury in people’s mouths,” said Dr. David Kennedy of San Diego, one of the organizers of a recent conference on mercury toxicity and vice president of the International Academy of Oral Medicine and Toxicology. “If they were not allowed to do that, the problem would go away. Just like smoking is a personal choice, whether or not you have mercury fillings in your mouth should be a personal choice.”

Not that amalgam fillings pose a threat to everyone. The ADA claims that less than 1 percent of the population is “mercury sensitive.” But critics, who say the rate is far higher, estimate that the health of thousands of people like Yost has already improved once their fillings were removed.

Nonetheless, not even critics of amalgam fillings recommend removing them unless someone is ill or having unusual symptoms. Improper removal can expose patients to more mercury than if the fillings are left in place.

Amalgam critics do urge that no new silver fillings be put in.

“You have the right to know what goes into your mouth,” said Louise Herbeck of Downers Grove, Ill., who founded a support group for those suffering the effects of fillings. “Dentists are self-regulated, but this is one thing that people should be able to control.”

Called DAMS, for dental amalgam mercury syndrome, Herbeck’s support group has 200 paying members. She estimates that 3,000 people have contacted her with filling-related problems.

For many of them and for Herbeck, medical improvement after removal of mercury fillings was gradual. Yost’s case, because improvement was so immediate and dramatic, is expected to attract a lot of attention.