Receding gums are part of the problem. “When the gums recede the tooth root is exposed, which can make it more prone to getting decay,” says Sally Cram, DDS, a spokesperson for the American Dental Association. And if fillings fracture over time, bacteria can collect in the cracks and lead to more tooth decay under the old fillings.

Conditions like dry mouth can leave teeth more vulnerable to decay as well. “As your mouth dries out, its pH gets lower and the environment becomes more acidic, which is a ripe breeding ground for bacteria,” Cram says.

The best steps: To reduce the likelihood of cavities, brush and floss as recommended (see “A Smart Dental Routine,” below), and be especially diligent before bed, particularly if you snack at night. “Saliva [which helps protect teeth] naturally decreases as you sleep,” says Edmond Hewlett, DDS, a professor in the division of restorative dentistry at the UCLA School of Dentistry. “Anything you eat before bed—even just a cracker—will leave debris in your mouth that bacteria can feast on overnight.”

If you’re having pain, see your dentist as soon as possible. And if you’re especially cavity-prone, ask about prescription high-fluoride toothpaste for extra protection, Sheridan says. Your dentist may also recommend a custom tooth tray, which you fill with fluoride gel and place on your teeth for 5 minutes each night.