"If I don't stop smiling, I'm going to get premature wrinkles," coos an attractive blonde, her teeth glinting as she promotes a mail-order tooth-whitening kit called "Natural White" on a lengthy commercial made for cable television.

The business of whiter, brighter teeth is booming. Once the preserve of Hollywood actors who paid dentists handsome sums to cap their teeth, whiteners for home use have burst onto the market. In the past 18 months, more than a dozen products with similar names have appeared, all promising to combat yellow, stained teeth.

But the American Dental Association, which represents about 150,000 dentists, has expressed concerns about the safety of these products, saying there is too little scientific knowledge about them.

"At this point, the jury is still out," says Kenneth H. Burrell, director of the Council on Dental Therapeutics for the Chicago-based ADA.

"The notion that the products do in fact whiten teeth is beyond question. The question is, what does that mean down the road in terms of long-term health?" asks James H. Berry, executive editor of the Journal of the American Dental Association. The ADA has asked the Food and Drug Administration to investigate the safety of teeth whiteners and to determine whether they should be regulated as cosmetics or as drugs.

The Food and Drug Administration requires greater evidence of safety and effectiveness for drugs than for cosmetics. According to the FDA's definition, a drug is intended to affect the structure and function of the body, while a cosmetic is defined as anything "introduced into or otherwise applied to the human body for . . . promoting attractiveness or altering the appearance." In some cases, this distinction is blurred. The FDA regulates skin-bleaching products as drugs, for example, while hair bleaches are considered cosmetics.

There is disagreement about which category applies to teeth whiteners. To some degree, like various new anti-wrinkle skin creams, they seem to fall through a regulatory crack, as products that have been dubbed "cosmeceuticals" -- hybrids that appear to behave more like drugs than like traditional cosmetics.

Daniel Michels, director of the Office of Compliance in FDA's Center for Drug Evaluation and Research, says that his agency is currently reviewing possible safety issues as well as the regulatory status of the new tooth whiteners. Michels says the FDA has not received reports of injuries or adverse reactions from tooth whiteners. "An immediate hazard does not seem to be part of the problem," he says.

Manufacturers of the dental whiteners say that their products do not need FDA review or approval because they are cosmetics, not drugs, and contain many of the same ingredients that some mouthwashes do.

Most whiteners rely on chemicals known as "oxygenating agents" to bleach teeth. The most common ingredient is a 10 percent concentration of carbamide peroxide, which, in contact with mouth fluids, breaks down into hydrogen peroxide and releases a highly reactive form of oxygen.

In a recent statement, the ADA's Council on Dental Therapeutics said it was concerned about safety because some scientific studies have suggested that, under some circumstances, oxygenating agents could damage mouth tissues, delay healing, harm the pulp, or interior, of the teeth and even cause genetic mutations.

ADA officials noted, for example, that a Harvard University laboratory study published in 1986 in the Journal of Periodontology found that hamsters orally exposed to hydrogen peroxide alone developed precancerous growths. When hydrogen peroxide was administered along with a known cancer-causing chemical, as might be found in tobacco smoke, it appeared to increase the likelihood of cancer. Gerald Shklar, a Harvard dentist who participated in the research, said that implications for human use were uncertain and he was not concerned about low-dose exposure to hydrogen peroxide.

Various forms of hydrogen peroxide have long been used in oral health care products for cleansing and treating mouth sores. While such products are generally for occasional use for short periods, the new whiteners may involve "long hours of unsupervised exposure for long periods of time," warns Christine Dumas, a California dentist and consumer adviser to the ADA. Dumas says she's also concerned about "self-prescribed overuse by patients who want their teeth even whiter."

Dumas and other dentists use an older in-office bleaching procedure that uses a more concentrated form of hydrogen peroxide, in which teeth to be whitened are surrounded by a rubber covering intended to protect sensitive gum and mouth tissues. The ADA has endorsed this approach, but it can be time-consuming, expensive and is sometimes uncomfortable.

The ADA has asked manufacturers of the newer whiteners to provide data on safety and effectiveness. So far, says Burrell, makers of only three products -- EpiSmile toothpaste, Rembrandt and BriteSmile -- have provided information, none of it sufficient to assuage the ADA's concerns. The dental group says it is looking for safety studies of at least six months' duration and for evidence from well-defined studies showing effectiveness in order to evaluate individual products.

Proponents say that, in general, the new bleaching products appear to be most effective in whitening teeth with mild to moderate stains from cigarettes, coffee, food or aging, but acknowledge that long-term results are unknown. Severe discoloring from early exposure to the antibiotic tetracycline is less likely to be affected.

While some of the new products are commercially available to the public, others are sold only to dentists who are supposed to supervise their use by patients.

A 1989 case report published in a dental journal by University of North Carolina dental researchers showed that one patient who wore a fitted mouthguard containing peroxide gel for eight hours nightly over a five-week period achieved whiter teeth with "no long-term detrimental effects upon the teeth." They said the result was "typical of cases treated in this manner."

This approach was adapted for several new commercial products, including the Rembrandt whitening system used by about 30,000 dentists, according to its manufacturer, Den-Mat Corp. of Santa Maria, Calif.

Den-Mat President Robert L. Ibsen says his company has funded its own research involving about six studies and 200 patients at universities around the world. "There was an improvement in every patient to a greater or lesser degree," says Ibsen. Some patients are sensitive to the gel, he notes, which is why it is available only through dentists. Treatment is not cheap: a dental consultation costs between $50 and $100 and each fitted mouthguard costs about $200.

Den-Mat executives and ADA officials agree that there are more safety concerns about selling tooth whiteners directly to consumers. But Ronald K. Murayama, a California dentist who developed Natural White for home use, says he believes such worries are primarily economic. "I'm potentially taking money out of dentists' pockets," he says.

Both Murayama and Ibsen say they contacted the FDA before marketing their teeth whiteners and were told their products would be considered cosmetics, which do not require pre-market approval.

Natural White uses a mildly acidic pre-rinse, a peroxide bleaching gel and a white tooth polish that is applied for about three minutes daily for a month followed by indefinite monthly maintenance. Murayama says his product was tested for one month in 47 patients in his dental practice and was effective for 75 percent of them. "We haven't had any adverse problems," he says.

Murayama says he believes that ADA's safety concerns are valid and that tooth whiteners should be studied to determine "the long-term effects, if there are any." He says his firm will provide data from a study underway to the ADA when it is complete. Natural White promotional materials say the product has been used by more than 300,000 people.

FDA officials say the agency's initial review of home-use tooth whiteners is unlikely to be completed until 1991. Should the FDA discover that their products are harmful or make false therapeutic claims, they could be subject to regulatory action.