The bristles of a toothbrush are too thick to fit into the contact points between teeth — the places where two teeth touch each other. Plaque, food particles, and bacteria accumulate in these spaces every time you eat. Without flossing, this buildup remains undisturbed for hours, days, or even weeks, giving bacteria a protected environment where they can thrive, produce acids, and cause damage.

Interproximal cavities — cavities that form on the side surfaces of teeth where they contact each other — are among the most common types of tooth decay in adults. These cavities often develop silently, without pain or visible signs, until they are large enough to show up on a dental X-ray or cause sensitivity. By the time you feel a cavity between your teeth, it has usually penetrated through the enamel and into the softer dentin beneath, requiring a filling or sometimes a more extensive restoration.