A dental cavity is a permanently damaged area in a tooth’s hard surface that develops into a tiny hole. Cavities are the single most common chronic disease in both children and adults. Nearly 21% of American adults between ages 20 and 64 have at least one untreated cavity right now, according to CDC surveillance data from 2024. Understanding how cavities form, what they feel like at different stages, and how they’re treated can help you catch them early, when the damage is easiest to fix.
How a Cavity Forms
Your mouth is home to hundreds of species of bacteria. Some of them, particularly one called Streptococcus mutans, feed on sugars and starches left on your teeth after you eat. As these bacteria digest carbohydrates, they produce lactic acid as a byproduct. That acid pools against the tooth surface inside a sticky film called plaque.
Your tooth enamel is made of a mineral crystal called hydroxyapatite. It’s extremely hard, but it has a vulnerability: acid dissolves it. When the pH on the tooth surface drops below roughly 5.5 (and in some people, as high as 6.5 depending on their saliva chemistry), minerals start leaching out of the enamel. This process is called demineralization, and it’s where every cavity begins.
Normally, your saliva works to neutralize acids and redeposit calcium and phosphate back into the enamel. This tug-of-war between mineral loss and mineral repair happens all day. A cavity forms when the balance tips toward loss, usually because of frequent sugar exposure, poor brushing habits, or dry mouth that reduces saliva’s protective effect.
What Happens Inside the Tooth
A cavity doesn’t appear overnight. It progresses through distinct layers of the tooth, and the symptoms change at each stage.
Stage 1: White Spot on the Enamel
The earliest sign is a chalky white spot on the tooth surface where minerals have started to dissolve. At this point, there’s no hole yet, no pain, and the damage is actually reversible. Fluoride treatments and improved oral hygiene can help the enamel rebuild itself before a true cavity develops.
Stage 2: Enamel Breakdown
If demineralization continues, that white spot may darken to brown, and the enamel weakens enough to form a small hole. You still might not feel anything because enamel has no nerves. This is why routine dental checkups matter: cavities at this stage are painless but clearly visible on an X-ray or during an exam.
Stage 3: Decay Reaches the Dentin
Beneath the enamel sits a softer, yellowish layer called dentin. Once decay crosses the boundary between enamel and dentin, things accelerate. Dentin is less mineralized and contains tiny tubes that connect to the tooth’s nerve center. This is when you’ll likely notice sensitivity to hot, cold, or sweet foods and drinks. The tooth may also look visibly discolored.
An important detail: bacteria can’t actually invade the dentin as long as the enamel above it remains structurally intact, even if that enamel is weakened. But once the enamel physically breaks down, bacteria flood into the dentin and create a soft, moist zone of destruction that spreads much faster than the original enamel decay.
Stage 4: Pulp Involvement
At the center of every tooth is the pulp, a soft tissue packed with blood vessels and nerves. When decay reaches the pulp, it swells. Because the pulp is enclosed in a rigid shell of tooth structure, there’s nowhere for the swelling to go, so pressure builds on the nerve. The result is a persistent, sometimes throbbing toothache that can wake you up at night.
Interestingly, the pulp begins responding long before bacteria reach it. Slowly advancing decay triggers the tooth to lay down a defensive layer of new dentin (called tertiary dentin) to wall off the threat. Fast-moving decay overwhelms this defense, and the pulp can die entirely.
Stage 5: Abscess
If a dead or infected pulp goes untreated, bacteria can push past the root tip and form a pocket of pus called an abscess. Symptoms at this stage include severe pain radiating into the jaw, swelling of the gums or face, fever, and swollen lymph nodes in the neck. A tooth abscess is a medical concern because the infection can spread to the jaw, other parts of the head and neck, or even enter the bloodstream and cause sepsis.
Risk Factors That Speed Things Up
Frequent snacking on sugary or starchy foods keeps acid levels high throughout the day, giving your saliva less time to repair the damage. Sugary drinks are especially problematic because they bathe all of your teeth in sugar simultaneously. But it’s not just candy and soda. Crackers, chips, and dried fruit cling to tooth surfaces and feed bacteria for extended periods.
Dry mouth, whether from medications, medical conditions, or simply not drinking enough water, removes one of your best natural defenses. Saliva doesn’t just rinse food particles away; it actively delivers calcium and phosphate to rebuild weakened enamel.
Location matters too. The back teeth (molars) have deep grooves and pits on their chewing surfaces where food and bacteria collect. They’re harder to brush thoroughly, which is why most cavities form there. Children and teenagers are at particular risk: about 18% of children aged 6 to 8 have untreated decay in their baby teeth, and roughly 10% of adolescents have untreated cavities in their permanent teeth.
How Cavities Are Treated
Treatment depends entirely on how far the decay has progressed.
If the damage is still at the white-spot stage, no drilling is needed. Your dentist may apply a concentrated fluoride varnish or gel to help the enamel remineralize. Fluoride works by swapping into the mineral crystal structure of the tooth, creating a compound called fluorapatite that resists acid far better than the original enamel. Regular hydroxyapatite starts dissolving at a pH of about 5.5, but fluorapatite holds up until the pH drops below 4.0. That difference gives your teeth a meaningful buffer against acid attacks between meals.
Once a true hole has formed, the damage is permanent and needs a filling. The dentist removes the decayed portion of the tooth and fills the space with a restorative material. The most common options today are composite resin (tooth-colored) and amalgam (silver-colored metal). Composite fillings typically last 5 to 10 years, while amalgam fillings average 10 to 15 years. Ceramic and gold restorations last longer (10 to 15 years and 20 to 25 years respectively) but cost more.
When decay is extensive enough that a filling can’t support the remaining tooth structure, a crown covers and protects what’s left. If the pulp is infected or dead, a root canal removes the damaged tissue from inside the tooth before it’s sealed and capped. And if the tooth is too far gone to save, extraction is the last resort, sometimes followed by an implant or bridge to fill the gap.
Prevention That Actually Works
Fluoride is the single most evidence-backed tool for cavity prevention. Community water fluoridation alone reduces decay rates by 30% to 50%. Fluoride toothpaste, used twice a day, provides a topical dose directly to the tooth surfaces where it’s needed most.
Dental sealants are another highly effective option, especially for children. These are thin plastic coatings painted onto the chewing surfaces of molars, filling in the grooves where bacteria tend to hide. They act as a physical barrier between plaque and the tooth surface.
Beyond fluoride and sealants, the basics hold: brush twice daily, floss once a day to clean the surfaces between teeth that your brush can’t reach, and limit how often you snack on sugary or starchy foods. It’s not just the amount of sugar that matters but the frequency. Five pieces of candy eaten over five hours creates five separate acid attacks on your teeth. The same five pieces eaten in one sitting produces one.
Regular dental visits, typically every six months, let your dentist catch early white-spot lesions before they become full cavities. That’s the stage where the process is still reversible, and treatment is as simple as a fluoride application rather than a drill.