A cavity under a filling means decay has developed again in a tooth that was already treated. This is called recurrent or secondary decay, and it’s actually the most common reason fillings need to be replaced. The process works much like the original cavity did, but it’s harder to detect because the filling hides what’s happening underneath.
How Decay Forms Under a Filling
Over time, the seal between a filling and your natural tooth can weaken. Tiny gaps open along the edges where the filling meets the tooth surface, and bacteria slip into those spaces. Once inside, they produce acid that eats away at the tooth structure, just as they would on an exposed tooth. The bacteria involved are the same species responsible for regular cavities.
This can happen with any type of filling material. Composite (tooth-colored) fillings typically last 5 to 10 years, while amalgam (silver) fillings last roughly 10 to 15 years. That doesn’t mean decay is inevitable at those ages, but the longer a filling has been in place, the more likely its margins have started to break down. Chewing forces, temperature changes from hot and cold foods, and grinding your teeth all contribute to that gradual deterioration.
Decay under a filling can also develop quickly if the filling chips, cracks, or partially breaks loose, since a damaged restoration creates an obvious entry point for bacteria.
What It Feels Like
The frustrating part is that early decay under a filling often causes no symptoms at all. The filling itself acts as a barrier between the cavity and your nerve, so you may not feel anything until the decay has grown significantly. When symptoms do appear, they typically include:
- Sensitivity to hot, cold, or sweet foods and drinks
- A dull ache or sharp pain in the tooth
- Pain when biting down
- Visible discoloration around the edges of the filling, often brown or dark
If the filling is on a back tooth, you might notice food getting caught in spots where it didn’t before. That can signal a gap has opened at the margin. Some people also notice the filling feels loose or slightly different when they run their tongue over it.
How Dentists Find It
Because you can’t always see or feel what’s happening under a filling, detection usually happens during a routine dental visit. Dentists use two main approaches: visual inspection and X-rays, specifically bitewing radiographs that show the area between teeth where fillings commonly sit.
Neither method is perfect on its own. Research comparing visual inspection to X-ray findings shows surprisingly low agreement between the two, with a statistical agreement score of just 0.19 (on a scale where 1.0 would mean perfect agreement). That means what looks fine on the surface might show decay on an X-ray, and vice versa. Dentists get the most reliable picture when they combine both methods. Fillings with visible damage or discoloration at the margins are much more likely to have deeper decay confirmed by X-ray, with nearly 10 times the odds of having a significant lesion underneath.
This is one reason regular dental checkups matter even when nothing hurts. Catching decay early, before it reaches deeper tooth structure, keeps treatment simpler.
What Treatment Looks Like
The treatment depends on how far the decay has spread. Your dentist will remove the old filling, clean out all the decayed tooth structure, and then decide what comes next based on how much healthy tooth remains.
If the decay is small and caught early, a new, slightly larger filling can replace the old one. This is the simplest outcome and feels much like getting a filling for the first time.
If a significant amount of tooth structure has been lost, a filling alone won’t be strong enough. In that case, you’ll likely need a crown, which is a cap that covers the entire visible portion of the tooth. Crowns cost between $1,100 and $2,000 out of network on average, though insurance often covers a portion.
When decay has reached the pulp (the soft tissue inside the tooth containing nerves and blood vessels), you’ll need a root canal before the tooth can be restored. This involves removing the infected pulp, cleaning the inner chambers, and sealing them. A crown is almost always placed on top afterward to protect the weakened tooth.
In severe cases where the tooth is too damaged to save, extraction becomes the only option. This is relatively rare if the problem is caught at a reasonable stage, but it’s the reality when decay has been silently progressing for a long time.
What Happens If You Ignore It
Untreated decay under a filling follows a predictable path, though it moves slowly. Dental decay typically takes several months to reach the pulp. Once it does, you’ll develop pulpitis, an inflammation of the pulp that causes poorly localized, throbbing pain. You might not be able to tell exactly which tooth is the problem at that stage.
If the pulp dies, the pain temporarily disappears, which can trick people into thinking the problem resolved itself. It hasn’t. Bacteria continue spreading, and when an acute abscess forms at the root tip, severe, well-localized pain returns. At that point you’re dealing with an infection that can spread to surrounding bone and soft tissue. Antibiotics alone won’t fix a dental abscess. They can manage the infection temporarily, but without removing the source of decay, the problem always returns.
The progression from “I think something feels off” to a serious infection usually involves multiple episodes of intermittent pain along the way. Each of those episodes is a signal worth acting on.
Keeping Your Fillings Healthy Longer
You can’t prevent fillings from aging, but you can slow the process and reduce the risk of new decay forming at their margins. Fluoride is the single most effective tool here. It prevents mineral loss from tooth enamel, replaces minerals that have already been lost, and reduces the ability of bacteria to produce the acid that causes cavities. Brushing with fluoride toothpaste twice a day and drinking fluoridated water provide a baseline level of protection. If you’re especially cavity-prone, your dentist may apply a concentrated fluoride varnish during visits or recommend a prescription-strength fluoride rinse for home use.
Beyond fluoride, the basics matter more than they might seem. Flossing cleans the margins of fillings between teeth, exactly where recurrent decay most often starts. Limiting sugary and acidic foods and drinks reduces the acid attacks your fillings and teeth endure throughout the day. And regular dental cleanings remove hardened plaque (tarite) that you can’t get off with a toothbrush, giving your dentist a chance to inspect filling margins and catch problems while they’re still small.
If you clench or grind your teeth, a night guard can reduce the mechanical stress that wears fillings down prematurely. Mention it to your dentist if you wake up with jaw soreness or notice flat, worn spots on your teeth.