Cavities Explained: What a Cavity Is, How Dental Cavities and Tooth Decay Develop, the Symptoms to Watch For, and How a Dentist Treats and Prevents Dental Decay

What is a cavity? Dental cavities, dental decay, and what happens inside your tooth

Put simply, cavities are holes in the teeth. A cavity is a hole in a tooth caused by permanent damage to its hard surface, and the disease process behind it is properly called dental caries, or tooth decay. Dentists also use the term dental decay for exactly the same thing, so if two clinicians describe your situation differently, they are almost certainly describing one condition.

The damage starts in the enamel, the hard outer layer of your teeth that shields everything underneath. Enamel is the hardest substance in the human body, but it dissolves in acid. Bacteria in your mouth live on the tooth surface, feed on sugar and starch, and release acid as a waste product, and repeated acid attacks strip calcium and phosphate out of the enamel crystal structure faster than saliva can put them back. When enough tooth material has been lost, the surface collapses and a hole opens.

From there, decay works inward. Beneath the enamel is dentin, softer and threaded with microscopic tubules that connect to the nerve. Deeper still, inside your tooth, sits the pulp, which contains the nerve and blood supply and runs down into each tooth root. The inside of the tooth is where decay stops being silent and starts being painful. Dental cavities can form on any surface — the pits and fissures applied to the chewing surfaces of a molar, the smooth sides between teeth, and the exposed root surface around the tooth in older adults. Where these lesions appear, the areas of tooth decay are simply called cavities, and the pattern is different at different ages.

How common are cavities, and who gets them?

Cavities are more common than any other chronic disease of childhood, and prevalence stays high across adult life. In the United States, national surveys consistently find that the large majority of adults have had at least one cavity, and roughly a quarter of adults have untreated tooth decay at any given moment. Some people get cavities repeatedly despite good habits, because saliva flow, enamel structure, and diet all make cavities easier or harder to start. The American Dental Association and federal health information sources both note that untreated decay is concentrated in groups with limited access to routine dental care rather than spread evenly through the population.

Cavities in children deserve particular mention. Baby teeth have thinner enamel, so decay moves through them faster, and losing a baby tooth early can push the permanent tooth into the wrong position. Our guide to the care of the deciduous teeth covers what changes in the first years.

Cavity symptoms and causes: what starts the process

Cavities are caused by a chain reaction involving bacteria, sugar, and acid. Bacteria gather in a sticky film of dental plaque that reforms on clean teeth within hours. When you eat or drink something containing sugar or starch, those bacteria use it as fuel and produce acid as a byproduct.

The acid then pulls calcium and phosphate out of the enamel. Every sugar exposure triggers an acid attack lasting roughly twenty minutes, and it is the frequency of exposures, not the total quantity of sugar, that does the damage. Someone sipping a sweetened drink over three hours does far more harm than someone eating dessert in five minutes. Saliva neutralizes acid and redeposits minerals between attacks, which is why dry mouth is such a powerful risk factor.

Several things lead to tooth decay more readily and raise the risk of tooth decay overall:

1. Frequent snacking or sipping sugary drinks throughout the day, which never lets the mouth recover.
2. Poor dental hygiene, where inadequate brushing and flossing let plaque mature and mineralize into tartar.
3. Deep grooves in molars that trap food beyond the reach of a brush bristle.
4. Dry mouth, whether from medication, radiotherapy, or a medical condition.
5. Too little fluoride, which otherwise makes cavities much harder to start.
6. Receding gums, exposing root surfaces that have no enamel at all and decay quickly.
7. Acid reflux or frequent vomiting, which bathes the teeth in stomach acid directly.

How a dentist diagnoses tooth damage

At the dental office the examination combines several methods. A dental professional inspects each surface under good light, uses a probe on suspicious pits, and takes bitewing X-rays, which are the only reliable way to see decay between teeth or beneath an existing filling. Some practices add laser fluorescence or transillumination for early lesions. Every finding is charted so the same tooth can be compared at your next visit — small lesions that are not progressing are often monitored rather than drilled.

How can you prevent tooth decay and protect your oral health?

Prevention is far easier than repair, and the measures that work are unglamorous.

Daily habits that help prevent cavities

Brush your teeth at least twice a day with a fluoride toothpaste, for two full minutes, and do not rinse with water afterwards — spitting alone leaves a protective film of fluoride on the enamel. Our guide to brushing technique explains the mechanics.

Clean between your teeth once a day with floss or interdental brushes, because roughly a third of all cavities start on surfaces a toothbrush never touches. Reduce the number of sugar exposures rather than agonizing over the amount; water and unsweetened drinks between meals do more good than any product.

Ask about dental sealants. A sealant is a thin resin coating applied to the chewing surfaces of the back teeth, sealing the grooves against food and bacteria. Sealants cut decay in those surfaces substantially and are especially worthwhile in children, because they help prevent cavities in exactly the sites where a brush is least effective. Community water fluoridation and fluoride varnish both work through the same mechanism at a population level.

Finally, keep up regular dental checkups. Professional cleaning removes hardened deposits that home care cannot, and a professional teeth cleaning appointment is also when small lesions get caught. Regular dental care at home combined with professional review is the most reliable route to good dental health.

Frequently asked questions about cavities

Can a cavity heal on its own?

Only at the very earliest stage. A white spot lesion, where minerals have left the enamel but the surface is still intact, can remineralize with fluoride and improved cleaning. Once the surface has broken and a hole has formed, the tooth cannot rebuild that structure, and a filling is needed.

Does getting a filling hurt?

The tooth is numbed with local anesthetic, so the procedure itself is not painful. You may feel pressure and vibration. Mild sensitivity to cold for a few days afterwards is normal, particularly with deep fillings, and usually settles within two weeks.

How long can I leave a cavity before it becomes a problem?

There is no safe interval. Progression rates vary widely — from months in a child's baby tooth to years in a slowly advancing adult lesion — and the only way to know which you have is to be examined. Waiting turns a filling into a root canal, and a root canal into an extraction.

Are cavities contagious?

The bacteria involved, particularly Streptococcus mutans, can be transferred through saliva, which is why sharing spoons with an infant or cleaning a pacifier in your own mouth is discouraged. Whether a cavity actually develops still depends on diet, cleaning, fluoride, and saliva flow.

Do I need to fix a cavity in a baby tooth?

Usually yes. Baby teeth hold space for the permanent teeth, and early loss causes crowding. Untreated decay in a baby tooth can also infect the developing permanent tooth beneath it and is painful for the child in the meantime.

Does sugar-free mean cavity-free?

Not automatically. Sugar-free products avoid the fermentable sugars that bacteria use, but many are still acidic — diet sodas and sparkling waters with citric acid erode enamel directly, without any bacterial step. Xylitol-sweetened gum is the exception that actively helps, by stimulating saliva.

Can whitening or a filling change the color of a dark spot?

A dark spot that is active decay must be removed, not covered. Once the decay is cleared, a tooth-colored composite restores appearance. Whitening does not remove decay and should never be used to disguise it.

How often should I have X-rays for cavities?

Intervals are set individually by risk. A patient with several recent cavities may need bitewings every six to twelve months; someone decay-free with good hygiene may go two to three years. Your dentist should be able to explain the reasoning for your interval.

Medically reviewed: This article was reviewed for medical accuracy by dentists within the Leading Implant Centers network. It provides general information and does not replace an individual examination and diagnosis.