What Happens When You Have a Cavity?

A cavity means decay has weakened and broken part of a tooth’s hard structure, leaving a hole. It may be tiny, hidden between teeth or painless—but it is not simply a stain, and the missing tooth structure does not grow back.

Before a hole forms, however, decay begins as mineral loss from enamel. That distinction matters: an intact early lesion may be stopped or reversed with fluoride and changes that reduce repeated acid attacks, while an established cavity generally needs dental repair. The National Institute of Dental and Craniofacial Research (NIDCR) describes this boundary clearly: enamel can repair early mineral loss, but continued loss eventually weakens and breaks down the surface (NIDCR).

What is happening inside the tooth

Decay-causing bacteria use sugars and starches from food and drink to produce acids. Repeated acid exposure removes minerals from enamel. At first, this may create a chalky white area without a hole or symptoms.

If mineral loss continues, the enamel surface can collapse. Once decay passes through enamel into dentin—the softer layer underneath—it can spread more quickly toward the pulp, the central tissue containing nerves and blood vessels (MSD Manual).

The practical progression is:

1. Early enamel mineral loss: The surface remains intact. This stage may be arrested or remineralized.
2. A cavity in enamel or dentin: Tooth structure has been lost. The hole may trap plaque and food and usually needs restoration.
3. Decay near or in the pulp: The pulp can become inflamed or infected. Pain may linger after hot or cold exposure, occur without a trigger or worsen with biting—but symptoms alone cannot establish the depth.
4. Infection beyond the tooth: An abscess, or pocket of pus, may form around the tooth or gum. This can cause severe pain, swelling, fever or a bad taste.

These are useful biological boundaries, not a home grading system. A dentist may use an examination and, when appropriate, X-rays to find decay and judge its depth; some cavities cannot be seen in a mirror (NIDCR). For more on that process, see how a cavity is checked.

A cavity may not hurt

Early tooth decay usually causes no symptoms. As it advances, possible signs include toothache and sensitivity to sweet, hot or cold foods and drinks. Infection may cause pain, facial swelling and fever (NIDCR).

Pain is therefore not a reliable test of whether a cavity exists or needs care. A painless tooth can still have a cavity, while sensitivity can also come from erosion, exposed dentin, a crack or another problem. Likewise, a dark mark is not proof of decay; stain and cavity can look alike.

Pain can also change as decay progresses. If badly damaged pulp tissue dies, pain may temporarily stop even though the tooth has not healed; infection can subsequently develop near the root (MSD Manual).

What treatment may involve

Treatment depends on whether the surface is intact, how deep the decay extends and how much sound tooth remains:

• Intact early decay: Professional fluoride treatment and preventive changes may help stop or reverse mineral loss.
• A formed cavity: A dentist commonly removes or manages the affected tissue and seals the area with a filling. Modern treatment does not always require removing every trace of softened tissue; an ADA clinical guideline supports selective removal in many moderate or advanced lesions to preserve tooth structure and reduce outcomes such as pulp exposure (American Dental Association).
• Pulp involvement: Root canal treatment may be needed to remove diseased pulp and retain the tooth.
• A tooth that cannot be restored: Extraction may be necessary.

The NHS summarizes the same treatment escalation: fluoride for early decay, a filling for a hole, root canal treatment when decay reaches the pulp and, in some cases, extraction (NHS). A dental assessment—not pain level alone—determines which category fits.

Brushing with fluoride toothpaste and cleaning between teeth remain useful because they reduce the conditions that allow further decay. They do not refill an existing hole. Until the tooth is assessed, avoiding repeated sugary snacks and drinks may reduce additional acid attacks, but it is not a substitute for repair.

When not to wait

Arrange a dental assessment for a suspected hole, recurring sensitivity or toothache even if the problem is manageable. Ask for urgent dental care if there is facial or jaw swelling, intense tooth or gum pain, fever, a bad taste with swelling, or difficulty opening the mouth.

Get emergency medical help if swelling makes it hard to breathe, speak or swallow, if there is major swelling in the mouth, or if an eye becomes swollen or painful or vision changes (NHS). Emergency service routes and numbers vary by country.

A cavity does not automatically mean a root canal or tooth loss. The key issue is how far the process has reached: early mineral loss may still be reversible, but a true hole requires assessment because brushing cannot rebuild the missing part.