Cavities are holes in your teeth that grow deeper over time, destroying healthy tooth structure layer by layer. Left alone, a small cavity can progress from a painless surface spot to a deep infection that threatens the tooth entirely. About 21% of American adults between 20 and 64 have at least one untreated cavity right now, making this one of the most common chronic health problems in the country.
How a Cavity Starts
Your mouth is home to bacteria that feed on sugars from the food you eat. As they digest those sugars, they produce acid as a byproduct. This acid collects in the sticky film of plaque that coats your teeth, and when the acidity drops below a critical threshold (a pH of about 5.5), it starts pulling calcium and phosphate minerals out of your enamel. This process is called demineralization.
In the earliest stage, you won’t feel anything. The only visible sign is a white spot on the tooth’s surface, an area where minerals have been stripped away but no actual hole has formed yet. This stage is still reversible. Fluoride, saliva, and good brushing can help push minerals back into the weakened enamel and repair it before real damage sets in. Once demineralization outpaces your mouth’s ability to repair itself, though, the enamel breaks down and a physical hole forms. That’s the cavity.
What Happens Inside the Tooth
A tooth has three main layers, and a cavity works through them in sequence. Understanding this progression explains why a cavity that seems minor can become a serious problem.
Enamel: The outermost layer is the hardest tissue in your body. Cavities move relatively slowly through enamel, and you typically feel no pain at this stage. You might notice the white spot darken to brown as the enamel continues to break down.
Dentin: Beneath the enamel sits dentin, a softer, yellowish layer. Once decay reaches dentin, it accelerates because this tissue is less resistant to acid. Dentin contains tiny fluid-filled tubes that connect to the nerve inside the tooth. When a cavity exposes these tubes, temperature changes, sugar, and even cold air can cause fluid movement inside them, triggering a short, sharp jolt of pain. This is usually the stage where people first notice something is wrong.
Pulp: At the center of the tooth is the pulp, which contains the nerve and blood supply. When bacteria from a cavity reach the pulp, they cause inflammation and infection. Early pulp inflammation can still be reversed with treatment, and you’ll recognize it by pain that starts when you eat or drink something cold or sweet and stops within a second or two of removing the trigger. Once the inflammation becomes severe, the pain changes character. It starts showing up on its own, without any trigger, or lingers for minutes after you eat. At this point, the tissue inside the tooth is dying, and it can no longer be saved with a simple filling.
Why Cavities Cause Pain
Small cavities in enamel are painless because enamel has no nerve endings. The sensitivity begins once dentin is exposed. The leading explanation for this pain is called the hydrodynamic theory: stimuli like cold drinks or sugary foods cause the fluid inside those tiny dentin tubes to shift, which activates nerve fibers deeper in the tooth. That’s why the pain feels sharp and immediate.
As decay deepens and bacteria reach the pulp, the pain shifts from stimulus-driven to constant or spontaneous. The nerve tissue swells inside a rigid shell of dentin with nowhere to expand, which cuts off its own blood supply. This creates intense, throbbing pain that can radiate into the jaw, ear, or temple. If the infection progresses further, a pocket of pus called an abscess can form at the root tip. Abscesses can cause facial swelling, fever, and swollen lymph nodes in the neck.
How Fast Cavities Progress
There’s no single timeline. A cavity’s speed depends on your diet, how much saliva you produce, your fluoride exposure, and how well you clean your teeth. In someone with good oral hygiene and a low-sugar diet, a cavity might take many months to work through enamel. In someone with poor hygiene or frequent sugar intake, it can progress noticeably in just a few weeks. The jump from dentin to pulp tends to happen faster than the initial enamel phase because dentin is so much softer.
Children are particularly vulnerable. Nearly 18% of kids aged 6 to 8 have untreated decay in their baby teeth. Even in baby teeth that will eventually fall out, untreated cavities can cause pain, infection, and damage to the permanent teeth developing underneath.
What Untreated Cavities Lead To
A cavity never heals on its own once an actual hole has formed. It only grows. The consequences of leaving it untreated follow a predictable path: increasing pain, infection, and eventually tooth loss.
When bacteria from a deep cavity spill into the pulp and create an abscess, the infection can spread into the jawbone and surrounding soft tissue. In rare but serious cases, dental infections can reach the bloodstream or spread to other parts of the head and neck. Chronic oral infections also contribute to ongoing inflammation in the body, which has been linked to broader health effects over time.
Even before it reaches that point, an untreated cavity changes how you eat. People with painful teeth tend to avoid chewing on that side, shift to softer (often less nutritious) foods, and may struggle with sleep if pain flares at night.
How Cavities Are Treated at Each Stage
The treatment depends entirely on how deep the cavity has gone, which is why catching it early makes a significant difference in what you’ll experience in the dental chair.
- White spot stage: No drilling needed. Fluoride treatments, improved brushing, and reducing sugar intake can remineralize the weakened enamel and stop the process before a hole forms.
- Enamel or shallow dentin cavity: A standard filling. The dentist removes the decayed material and fills the space with a composite or other material. This is a straightforward procedure, and the tooth keeps functioning normally.
- Large cavity with significant tooth loss: When too much tooth structure has been destroyed for a filling to hold, a crown (a cap that covers the remaining tooth) restores its shape and strength.
- Pulp involvement: If decay has reached the nerve and the inflammation is irreversible, a root canal removes the infected pulp tissue, and the tooth is sealed and typically capped with a crown. If the tooth is too damaged even for that, extraction is the remaining option.
The difference between a 20-minute filling and a root canal (or losing the tooth entirely) often comes down to timing. A cavity that would have been a simple fix six months ago can become a much bigger problem if ignored. Since early cavities are painless, regular dental checkups catch them at a stage where treatment is minimal and the tooth stays intact.