Then in 2006, she received a letter from a lawyer representing the state dental board. She was practicing dentistry without a license, the letter read, and must immediately “cease and desist” selling her products in Alabama.

Last month, Ms. Wilson and another owner of a teeth-whitening business sued in state court, arguing that a law declaring teeth whitening the exclusive province of dental professionals is unconstitutional.

Though Ms. Wilson did not know it when her trouble started, this dispute is one of many taking place all over the country between nondentist teeth whiteners and state dental boards. This issue is itself part a much broader debate over the proper limits of occupational licensing and the amount of leeway that professional boards should be given to set up barriers of entry.

“Licensing is a labor market institution that’s been growing dramatically,” said Morris M. Kleiner, a professor at the Humphrey School of Public Affairs at the University of Minnesota, who has written books and papers on the subject. Five percent of United States workers had jobs that required licenses in the 1950s, he has found, and that has risen to around 30 percent now.

For Ms. Wilson, though, this is just about teeth.

“Teeth whitening is for everybody,” she said, describing it in the context of permanent makeup, microdermabrasion and various other procedures that perhaps explain why she does not look 71 years old.

Teeth whitening began to flourish in the 1990s, with dentists using forms of peroxide to essentially bleach the teeth (as opposed to removing stains, which is part of routine cleaning). The service generally costs around $300 or more, according to court documents; by 2006, a survey by the American Academy of Cosmetic Dentists found that dental practices were making an average of $25,000 a year from teeth whitening.

Later, whitening strips or kits — which often consist of peroxide gel, an application tray and in some cases a little LED light that activates the gel — began appearing online and in pharmacies, where they can be bought over the counter like cosmetics.

Teeth whitening also began to be offered as a service in salons and spas and at mall kiosks. There, employees provide the devices and explain how to use them, but, such business owners emphasize, nobody puts hands or fingers into anyone else’s mouth. The services at salons and kiosks generally cost around $100. Altogether, teeth whitening has become a multibillion-dollar industry.

Starting in 2005, state dental boards began adopting polices and pushing for legislation that would make teeth whitening illegal for anyone to perform but dentists or dental hygienists. According to the Institute for Justice, a law firm that is taking up Ms. Wilson’s suit, at least 14 states have, by policy or statute, defined teeth-whitening services as the practice of dentistry.

The Institute for Justice has long been involved in campaigns against what it regards as overzealous licensing requirements, arguing that in areas like interior design or hair braiding, they are simply anticompetition measures. What sets the teeth-whitening debate apart, said Paul M. Sherman, an attorney at the institute, is that the same products prohibited for use in salons in Alabama and several other states can be bought over the counter in those states with no trouble at all.

“The equal protection argument is particularly compelling because these are the same products that people buy and use at home every day,” he said.
While the institute says there have been very few reported complaints from consumers about teeth whitening, dentists say there are real health concerns.

“A thorough oral examination, performed by a licensed dentist, is essential to determine if whitening is appropriate and whether the patient has other oral health issues that may affect his or her decisions about whitening or other treatments,” the American Dental Association said in a statement.

The association has also petitioned the Food and Drug Administration to take another look at “tooth-whitening agents” sold in stores and online; they are currently classified as cosmetics.

The crusade of teeth-whitening entrepreneurs has had some help from the Federal Trade Commission, which in 2011 ruled that North Carolina’s Board of Dental Examiners — which also sent a letter to Ms. Wilson — had illegally thwarted competition with its cease-and-desist letters.

But the situation in Alabama is somewhat different. In 2009, the State Supreme Court ruled that a dental service is any “helpful act or useful labor of or relating to the teeth,” and by this rather basic definition, teeth whitening is dentistry. 

Soon after, the Legislature passed explicit language to that effect. A nondentist administering teeth whitening could now face a misdemeanor punishable by up to a year in jail and a $5,000 fine.

“The board is formed to uphold the law,” said Susan Wilhelm, the executive director of the Alabama Board of Dental Examiners. “They are looking out for the health, safety and public welfare as they are directed by the Legislature.”

Ms. Wilson and her lawyers say the board is not simply carrying out law but helping make it, too. As far back as 2008, the board was considering legislative language that would explicitly prohibit anyone other than dentists from performing teeth whitening.

In any case, Ms. Wilson insists that she will not back down in her legal fight. She also added that she never liked going to the dentist anyway, and had only been twice in her life. Two fillings, she said, smiling.