Can a cavity form under an existing filling?
Yes. The filling material cannot decay, but the tooth around and beneath it can. New decay most often starts at the margin where filling meets tooth, especially near the gum line where plaque collects, and it can travel under the filling out of sight. It is often painless until it is deep, so it is usually found at a check-up by probing the margins and by x-rays rather than by symptoms.
How it forms
Plaque sits along the junction; a gap, a worn edge or a rough margin lets bacteria in; the same diet and dry-mouth risks that caused the first cavity continue. The Institute for Quality and Efficiency in Health Care describes how decay progresses through enamel and dentin (IQWiG).
What changes the risk
- Your overall decay risk: diet frequency, saliva, fluoride, cleaning between teeth (NIDCR)
- The filling's age, size and location; margins between teeth or at the gum are hardest to clean
- The material and how well it was sealed when placed
- Grinding, which opens margins over time
How it is found
Visual and tactile examination of margins, bitewing x-rays on a schedule set by your risk (ADA), floss catching, and sometimes light-based aids. A dark line at a margin is not always decay; staining and old material can look similar, and the examination tells which.
Symptoms, when they come
Sensitivity to sweet or cold at one tooth, a rough edge, food catching, a piece of filling or tooth breaking away. Pain that lingers or wakes you means the nerve may be involved.
What is done
A small lesion at the edge can sometimes be repaired by removing that part and adding new material; decay running under the filling means removing the filling and restoring the tooth again, sometimes with an onlay if the walls are now thin. The FDA notes that sound fillings do not need replacing (FDA).
Prevention
Fluoride toothpaste, cleaning between teeth daily, fewer sugar exposures, managing dry mouth, and keeping the check-up interval your dentist sets.
Next step: Request a consultation or call (805) 565-5111. A request is not a confirmed appointment; the office confirms times during its listed hours (Monday to Thursday, 8 am to 5 pm, plus select Fridays).
Margin-decay explainer
In words: a filling is a patch on a tooth. The patch cannot rot; the fabric around the stitching can.
| Where decay starts around fillings | Why | What helps |
|---|---|---|
| At the gum-line edge | Plaque collects | Brush at the gum line |
| Between teeth | Brush cannot reach | Floss or interdental brush daily |
| At a worn or rough edge | Bacteria enter the gap | Report catching; repair early |
| Under a leaking filling | Hidden | Risk-based x-rays |
Guides are general education; an examination is needed for individual advice.
Sources
- Overview: Tooth decay (InformedHealth.org, updated Sep 2023) — Institute for Quality and Efficiency in Health Care (IQWiG)
- Oral Hygiene — National Institute of Dental and Craniofacial Research
- Oral Health Topics: X-Rays/Radiographs (updated Mar 26, 2026) — American Dental Association
- Information for Patients About Dental Amalgam Fillings — U.S. Food and Drug Administration
Sources were current when this guide was last updated.
Educational content, not a diagnosis. Please contact the studio to discuss your own situation.