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Tooth Decay Under Filling

Updated: Nov 13, 2023

It is possible to get tooth decay under a filling because the decay can enter into the gap between the filling and your natural tooth structure. This condition is called recurrent tooth decay since it is not the first time that it happened.

There is a microscopic gap that may form where the filling meets the natural tooth structure. If a cavity were to form, the most likely point of entry would be that gap or junction between the filling and the tooth.

broken filling with cavity underneath
broken filling with cavity underneath

In case you were wondering if you have this condition it is called recurrent tooth decay, this article will explain everything that you need to know about it. We'll also let you know what you should do!Good oral hygiene

How to tell if you have tooth decay under a filling

The only way to tell if you have decay under a filling is to get it confirmed by a dentist. Nonetheless, if you have any of these signs or symptoms, you should go in for a check up because you may have it.


  • Sensitivity to sweets. You may get a zing or an unpleasant sensation if you eat anything sweet with sugar in it.

  • Discolored filling. Your tooth filling looks like it is changing colors. Usually if decay sneaks in under the existing filling, you'll see a black shadow coming through the restoration.

  • Bad smell. If you're getting a whiff of a bad small coming from your mouth, it could be a leaking filling with recurrent decay.

When the decay gets to the point where it can cause a hole next to the old filling, it will definitely be sensitive. The reason is because the dentin, which is much more sensitive than the enamel will be completely exposed to the outside elements. Of course this is all a part of the progression for the stages of tooth decay, small cavities progress through the tooth to become bigger ones.

Why does recurrent decay happen?

Recurrent decay under a filling can develop from new decay in the gap, de-bonding, or the existing filling breaking.

  • New decay. The filling itself may not be able to get a cavity but the natural tooth structure immediately next to it can. Decay can form at that junction where the filling meets the tooth.

  • De-bonding. The tooth colored fillings are bonded onto the tooth. If that bonding melts away, decay can sneak in and start forming.

  • Broken filling. A filling that is fractured or damaged is a prime spot for tooth decay to develop. This is why it is important to get any damaged restorations replaced as soon as possible.

Treatment for a cavity under a filling

Since there is decay under the filling, the cavity will need to be removed first and then replaced with a new filling. However, depending on the extent of the decay, a cavity filling may not be enough and you may need a crown or even a root canal sometimes.

  • Small decay under filling. Another dental filling should be sufficient to restore this tooth back to normal. Don't put it off because the cavity can grow and you may need to move onto the next treatment.

  • Medium size decay under filling. If the cleaned out cavity will encompass more than 50% of the entire tooth, you may need a dental crown. A crown will offer better structural integrity than a large filling. An analogy would be patching up your pants if there was a hole. However, if you were missing an entire leg on your pants you may want a new pair!

  • Large decay under filling. If the cavity is very big, it could very well be close to the nerve. In that case you will need to treat the nerve by removing it via a root canal procedure. Afterwards you can get a crown to protect the tooth.

How to prevent it from happening again

As you are now aware, fillings can get decay underneath them so just because you get a cavity filling, it does not make it immune to new decay. The only way to prevent it from happening again is to maintain good oral hygiene and minimize the sugar intake.

  • Good oral hygiene. If you want acknowledgement from your dentist, you must practice good oral hygiene. This means you should brush for at least 2 minutes twice a day and then floss before you go to bed.

  • Minimize sugar. Reducing or even eliminating sugar from your diet will do wonders to prevent cavities from coming back. The reason is because the bacteria which cause them require sugar as a fuel. If you deprive them of it, they won't be able to cause any damage to your teeth.


Yes, you can certainly get a cavity under a filling because new cavities can attack the gap in between the tooth and filling. Therefore, even after getting a cavity filling you should still be diligent with maintaining your oral hygiene and minimizing sugar intake. That is the only way to keep cavities away.

Author: Written by Dr David Chen, DDS a long island city dentist.



David Chen 200 x 200.jpg

About the author: Dr David Chen, DDS

Hello, I'm Dr Chen and I'm an actively practicing dentist in Long Island City, NY. I graduated from Columbia University College of Dental Medicine in 2016 but prior to going to dental school I was already working in the dental field. It's been more than a decade since I first got to know dentistry and let me tell you, time flies by quickly. Since then I've developed a fondness for writing, which is how this all got started!

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Medical Disclaimer:

This blog is purely meant for information purposes and should not be used as medical advice. Each situation in your mouth is unique and complex. It is not possible to give advice nor diagnose any oral conditions based on text nor virtual consultations. The best thing to do is to go in person to see your dentist for an examination and consultation so that you can receive the best care possible.

The purpose of all of this oral health information is to encourage you to see your dentist and to inform you of what you may expect during your visit. Due to the unfortunate nature of dentistry, there isn't really any true home remedies that will get rid of dental problems. Roughly 99.99% of them require in-person intervention by a healthcare professional.

Hint: That is the reason why you can't eliminate seeing dentists in your life!

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