According to the experts we spoke with, not everyone should be using over-the-counter whitening strips. "Anyone with sensitive teeth, gum disease, cavities, exposed roots, crowns, or fillings should avoid teeth whitening strips," says Dr. Wei. "People who are pregnant or nursing should also avoid teeth whitening strips."

Also, Dr. Kevin Sands, a board-certified cosmetic dentist, says that if you're allergic to peroxide, a main ingredient in most white strips, you should avoid them.

Plus, teeth whitening strips aren't great at addressing people with naturally grayer teeth, "tetracycline staining," a gray discoloration from childhood antibiotics. "The same applies to internal staining, such as a discolored tooth due to past trauma or an aging root canal," says Dr. Martin. "In these cases, a specialized procedure performed by an endodontist is necessary."