What Happens If You Get a Cavity Under a Filling?

Have you ever wondered what happens if you get a cavity under a filling? At first, it may seem impossible—after all, isn’t the filling supposed to protect the tooth? The truth is, decay can still sneak in underneath, often without obvious signs at first. What causes this hidden problem, and what happens if it’s left untreated? Let’s explore the answers.

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TL;DR:

Cavities can still develop under fillings due to gaps, wear, residual decay, or poor oral hygiene. Early signs include sensitivity, pain when chewing, discoloration, loose fillings, and gum tenderness. Dentists diagnose these hidden cavities using exams, X-rays, and advanced imaging. If untreated, decay can spread, cause infection, fractures, or even tooth loss—leading to more invasive, costly treatments. Options range from simple filling replacement to crowns, root canals, or extraction, with prevention key to long-term oral health.

What Happens If You Get a Cavity Under a Filling What Happens If You Get a Cavity Under a Filling?

What Causes Cavities to Form Under a Dental Filling?

A dental filling is meant to solve the problem of decay by removing the infected tissue, sealing the cavity, and restoring normal function. Yet, even after a filling, cavities can sometimes develop underneath or around it. This happens when the protective seal weakens or bacteria find ways to infiltrate the tooth again.

Several factors can contribute to this process. Over time, tiny gaps may form between the restoration and tooth surface due to wear, temperature changes, or material shrinkage—allowing bacteria to slip in and cause recurrent decay. In some cases, decay wasn’t fully removed during the initial procedure, giving bacteria a chance to persist and spread beneath the filling.

The type of material, how well it was placed, and the patient’s oral hygiene habits all play an important role as well. Fillings can also deteriorate naturally with age, or become damaged from chewing, grinding, or thermal stress. Together, these factors explain why cavities can still form under restorations and why ongoing care and monitoring are essential.

Signs and Symptoms of a Cavity Under a Filling

Cavities hidden beneath fillings are tricky to spot, but they often show subtle warning signs before becoming severe. These symptoms may start off mild and grow more noticeable as decay progresses.

Cold, heat, or sweet sensitivity – discomfort triggered by temperature changes or sugary foods.
Pain when chewing or biting – sharp or throbbing pain if the restoration or tooth structure is compromised.
Discoloration or dark spots near the filling – a shadow or stain may signal decay beneath.
Visible gaps, chips, or fractures – damage at the edges weakens the seal and invites bacteria.
Loose or shifting filling – movement of the restoration allows bacterial entry.
Bad taste or persistent bad breath – often from bacteria or food trapped under the filling.
Lingering or unexplained pain – dull aches, pain at night, or discomfort after hot/cold exposure.
Swelling or gum tenderness – advanced decay may irritate surrounding tissues.

How Your Dentist Diagnoses Cavities Under a Filling

When cavities develop beneath the restoration, they often remain hidden until the damage progresses. Dentists use a combination of clinical skills and diagnostic tools to detect these problems early and prevent them from turning into more serious complications. Here are the main methods they rely on:

Clinical examination – checking for visible gaps, discoloration, chips, or signs of wear around the repair.
Sensitivity and percussion tests – tapping on or applying pressure to the tooth to see if it causes pain, which may indicate deeper decay.
Biting or mobility tests – using tools to detect discomfort when biting or movement in the filling that signals underlying weakness.
Radiographs (X-rays) – one of the most reliable methods to spot decay beneath or around restorations, especially between teeth or under margins.
Transillumination or advanced imaging – shining light or using specialized diagnostic tools to reveal hidden cracks or shadowing from decay.

By combining these approaches, dentists can accurately determine whether a filling is failing or if decay has developed underneath. Early diagnosis helps avoid more invasive, costly treatments and preserves as much of the natural tooth as possible.

Potential Risks of Untreated Cavities Under a Filling

Leaving cavities under a repair untreated can have serious consequences, as decay continues to spread beneath the surface where it is harder to detect. What starts as a small hidden problem may eventually compromise both the tooth and the restoration.

Progression of decay into deeper layers – If untreated, decay can spread into dentin and eventually reach the pulp, leading to inflammation and the need for more complex procedures such as root canal therapy.
Infection and abscess formation – Once bacteria invade the pulp, infection may follow, sometimes creating an abscess at the root tip and even spreading to surrounding bone or tissues.
Tooth fracture and weakening – The decayed tooth structure under a repair becomes fragile, increasing the risk of cracks, fractures, or even loss of the filling itself.
More invasive treatments – Early replacement of a repair might be enough, but delayed treatment often leads to the need for crowns, root canals, or extractions.
Pain and reduced quality of life – Persistent discomfort, sensitivity, or difficulty chewing can interfere with eating, sleeping, and daily routines.
Systemic spread of infection – In severe cases, untreated dental infections may extend beyond the tooth, causing swelling, fever, and risks to overall health, especially in medically vulnerable individuals.
Increased cost and complexity – The longer treatment is postponed, the more extensive and expensive it becomes, while also reducing the chance of preserving the natural tooth.

Addressing decay beneath a filling early is the best way to protect both oral health and overall well-being, while minimizing discomfort, costs, and treatment time.

Treatment Options

When a cavity develops under a filling, treatment depends on how advanced the decay is, how close it is to the pulp, and the patient’s overall risk factors. In the earliest stages, when the decay is shallow and not causing symptoms, dentists may recommend non-invasive approaches. These include improving oral hygiene, applying fluoride varnishes or gels, using resin sealants, or re-sealing small gaps around the filling to halt further progression.

If the decay is more localized, the usual solution is to remove the existing filling, clean out the affected area, and replace it with a new restoration. In deeper cases where the cavity approaches the pulp but the pulp is still healthy, a protective liner such as calcium hydroxide or MTA may be applied before placing the filling. For teeth that have lost a significant amount of structure, a crown or onlay may be required to provide strength and restore proper function.

When decay reaches the pulp and causes irreversible damage, root canal therapy is often necessary to remove the infected tissue and save the tooth. Afterward, the tooth is typically reinforced with a crown. Nevertheless, if the tooth is too compromised to be restored, extraction may be the only option, followed by replacement solutions such as implants or bridges. Alongside treating the immediate problem, prevention remains critical—patients are encouraged to maintain strong oral hygiene habits, control sugar intake, use fluoride products, and attend regular checkups to reduce the risk of future issues.

Key Takeaways:

Cavities can still form under fillings when gaps develop, decay is left behind, materials degrade, or oral hygiene is poor.
Early signs and symptoms include sensitivity to hot, cold, or sweet foods, pain while chewing, discoloration near the filling, chips or fractures, looseness of the filling, bad breath, persistent pain, and gum swelling.
Dentists diagnose hidden decay using visual inspection, sensitivity and percussion tests, biting/mobility checks, X-rays, and advanced imaging techniques.
Untreated cavities under fillings can worsen, leading to deeper decay, pulp infection, abscesses, tooth fractures, more invasive treatments, systemic infection risks, pain, and higher treatment costs.
Treatment options depend on severity: from non-invasive fluoride or sealing for early cases, to replacement fillings, liners, crowns, root canals, or extractions in advanced cases. Preventive care and good oral hygiene remain essential to avoid recurrence.

Sources.

Ku, J. C., Lam, W. Y., Li, K. Y., Hsung, R. T., Chu, C. H., & Yu, O. Y. (2025). Accuracy of detection methods for secondary caries around direct restorations: A systematic review and meta-analysis. Journal of dentistry, 153, 105541. https://doi.org/10.1016/j.jdent.2024.105541

Nassar, H. M., & Yeslam, H. E. (2021). Current Novel Caries Diagnostic Technologies: Restorative Dentists’ Attitude and Use Preferences. Healthcare (Basel, Switzerland), 9(10), 1387. https://doi.org/10.3390/healthcare9101387

Ghodasra R, Brizuela M. Dental Caries Diagnostic Testing. [Updated 2023 Mar 19]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK574510/