It’s not fully clear, but once or twice a year seems to cause no problems, said Dr. Dan Nathanson, chair of the department of restorative sciences and biomaterials at the Boston University School of Dental Medicine.

“No one has shown categorically that there is a danger in doing it too much, but we are cautious. It’s probably not good to be a whitening addict,” he said.

The basic ingredient in tooth whitening gels, whether administered in a dentist’s office or at home, is hydrogen peroxide, which penetrates the porous enamel on the outside of the tooth and the inner layer, dentin, lightening the tooth as it goes.

Whitening does not seem to destroy either the enamel or the dentin. All forms of bleaching can cause temporary tooth sensitivity.

There are some differences — including financial — between whitening done at home and that done at the dentist’s office. In the office, dentists use a stronger concentration of peroxide gel, 30 to 35 percent, and the procedure can run about $1,500 for three treatments.

With the middle-priced option, a dentist takes impressions of both upper and lower teeth, which costs about $800, and makes retainer-like arches. The patient fills the arches with gel and inserts them every night for a set number of days.

Over-the-counter whitening strips, which cost roughly $30, use the weakest concentrations of peroxide, but still “do a pretty good job,” said Nathanson. The strips only reach from one canine (“eye”) tooth to the other “and most people have bigger smiles than that,” though, said Dr. Howard Strassler, professor of operative dentistry at the University of Maryland Dental School. And if your teeth aren’t perfectly straight, they may not bleach evenly.