Cavity Between Teeth: Why They Happen and How They’re Treated

Editorially reviewed by the Dental.me Editorial Team · Updated July 2026

A cavity between teeth, called an interproximal cavity, is one of the most common types of tooth decay and also one of the sneakiest. It forms in the tight contact area where two neighboring teeth touch, a spot your toothbrush bristles physically cannot reach. Because it hides in that gap, this kind of decay often grows silently for months or years before you notice anything at all. Understanding why these cavities happen, and why flossing matters more here than anywhere else in your mouth, is the key to catching and stopping them early.

What Is a Cavity Between Teeth?

An interproximal cavity is simply a cavity that develops on the side surface of a tooth, in the region where it meets the tooth next to it. Dentists call these touching surfaces the proximal or contact surfaces. When plaque acids dissolve the enamel there, decay starts on one tooth and, if left alone, spreads across the contact into the neighboring tooth as well.

What makes this location different from a cavity on the chewing surface is access. The tops of your molars have deep grooves you can scrub, but the flat sides pressed against a neighbor are sealed off from your brush. The only tool that reaches into that seam is floss or an interdental cleaner, which is exactly why between-teeth decay is so closely tied to flossing habits.

Why Cavities Between Teeth Are So Common

The contact area is the single spot brushing misses most. Food debris and plaque get packed into it every time you eat, and if that plaque is not removed daily it feeds the bacteria that produce enamel-dissolving acid. A few habits and conditions push the risk even higher:

• Not flossing. Brushing alone cleans roughly the front, back, and top of a tooth but leaves the two sides untouched. Skipping floss leaves those surfaces coated in plaque.
• Sugar and frequent snacking. Every sugary snack or sip restarts the acid attack. Frequency matters more than quantity, which is one of the main causes of cavities in hard-to-clean areas.
• Tight contacts that trap food. Teeth that press together firmly can wedge food fibers deep into the gap where they are hard to dislodge.
• Dry mouth. Saliva rinses away acid and delivers minerals back to enamel. Low saliva flow, whether from medication or a medical condition, removes that natural defense.
• Existing fillings or crowns with gaps. A slightly open margin at the edge of old dental work creates a ledge that traps plaque right at the contact point.

Why They Are So Hard to Spot

The frustrating thing about a cavity between teeth is that you usually cannot see it or feel it while it is still small. Enamel has no nerves, so early decay is painless, and the damage is tucked out of sight behind the contact point where no mirror can catch it. This is why so many people are surprised when a dentist points one out.

The reliable way to find interproximal decay is a bitewing X-ray. On the film, decay shows up as a darker shadow eating into the bright enamel between two teeth, often long before any symptom appears. That is the whole reason routine checkups matter: your dentist can catch and treat a small between-teeth cavity before it ever reaches the point of causing pain. If you want to know more about the warning signs, our guide on how to tell if you have a cavity walks through them in detail.

Symptoms once it advances

By the time a between-teeth cavity produces symptoms, it has usually grown well into the tooth. Watch for:

• Sensitivity to sweet, hot, or cold when it hits that area
• Floss that shreds, snags, or catches in one specific spot every time
• Food consistently packing into the same gap
• A dark or grayish shadow visible near the contact between two teeth
• A toothache or throbbing, which can mean the decay has reached the nerve

How a Dentist Treats a Cavity Between Teeth

Treatment depends entirely on how far the decay has spread by the time it is found, which is another argument for regular X-rays.

• Small cavity: a filling. A modest interproximal cavity is treated with a filling, usually tooth-colored composite. Because the decay sits on the side, the dentist typically drills in from the top or side of the tooth to reach it, then uses a thin metal matrix band wrapped around the tooth to rebuild the wall and restore a proper contact with the neighbor. You can read more about the process and typical cost of a dental filling in our dedicated guide.
• Larger cavity: an inlay, onlay, or crown. When decay has destroyed a bigger portion of the tooth, a simple filling may not hold. A lab-made inlay or onlay, or a full crown, restores strength.
• Decay reaching the pulp: a root canal. If the cavity has spread all the way to the nerve and blood supply inside the tooth, a root canal removes the infected tissue, and the tooth is usually crowned afterward.

Can an Early Cavity Between Teeth Be Reversed?

Sometimes, but only very early. The first stage of decay, before a hole forms, is a white-spot lesion where minerals have leached out of the enamel but the surface is still intact. Caught at that point, a white-spot lesion can often be arrested or even remineralized with fluoride, improved cleaning, and reduced sugar. Your dentist may recommend a prescription-strength fluoride and closer monitoring rather than drilling right away.

Once the enamel has actually broken down into a cavitated hole, however, that damage cannot heal itself. The bacteria are now sheltered inside a pit that no amount of brushing or fluoride can clean out, so a filling becomes necessary. This is why the honest answer is: the earlier it is found, the more options you have.

Why you should not wait

A between-teeth cavity does not stay put. It spreads sideways into the neighboring tooth, turning one problem into two, and it drills steadily inward toward the nerve. What could have been a small filling can become two fillings, or a root canal and a crown, in a matter of months. Treating early is almost always cheaper, faster, and less invasive.

How to Prevent Cavities Between Teeth

Prevention here is unusually straightforward because the cause is so specific. Clean the surfaces your brush cannot reach, and lower the acid load in your mouth:

• Clean between your teeth every day. Floss or use an interdental brush or water flosser once a day to remove plaque from the contact areas. If your technique feels awkward, our step-by-step guide on how to floss makes it easier.
• Use fluoride. Brush twice daily with fluoride toothpaste to keep re-hardening the enamel between acid attacks.
• Cut sugar frequency. Reducing how often you snack and sip sugary or acidic drinks gives your enamel time to recover between meals.
• Keep up regular X-rays and checkups. Bitewing X-rays at recommended intervals are the only way to catch between-teeth decay while it is still small and reversible.

When to See a Dentist

Book a visit if floss keeps catching or shredding in one spot, if food repeatedly packs into the same gap, if you notice new sensitivity or a dark line near where two teeth meet, or simply if you are overdue for a checkup. Because interproximal cavities are invisible and painless early on, the most important thing you can do is not wait for symptoms. Regular professional exams and daily cleaning between your teeth are what stop a small shadow on an X-ray from ever becoming a root canal.

Find care near you: compare dentists in your area, or browse dental specialties.