A cavity is a permanent hole in your tooth caused by decay, and what happens next depends entirely on how far it has progressed. About one in five adults between ages 20 and 64 has at least one untreated cavity right now. In its earliest stage, decay can actually be reversed. But once a true cavity forms, it will keep growing until it’s treated, potentially reaching the nerve inside your tooth and causing serious pain or infection.
How a Cavity Forms
Your mouth is home to bacteria that feed on sugars and starches from the food you eat. As they digest those carbohydrates, they produce lactic acid as a waste product. That acid can drop the pH in your mouth low enough to dissolve the minerals in your tooth enamel, the hard outer shell that protects each tooth. Enamel is made primarily of a calcium phosphate mineral, and acid pulls those minerals out of the surface in a process called demineralization.
Normally, your saliva works to repair this damage by depositing minerals back into the enamel. Fluoride from toothpaste speeds up that repair. But when acid attacks happen too frequently, your saliva can’t keep up. The enamel weakens, and eventually a physical hole forms. That’s the cavity. Once enamel has broken through, it cannot regrow on its own.
The Earliest Stage Is Reversible
Before a cavity actually forms, decay shows up as a white spot on the tooth surface. These white spot lesions represent mineral loss without a physical hole. They’re the first visible sign that something is going wrong, and they’re important because they can still be reversed. Improved brushing habits, fluoride treatments, and reducing sugar intake can allow your enamel to remineralize and recover. Your dentist may spot these during a routine exam and recommend a fluoride gel or varnish rather than a filling.
Once the surface breaks down into an actual cavity, though, that window closes. No amount of brushing or fluoride will fill a hole that has already formed in the enamel.
What It Feels Like at Each Stage
Early cavities often cause no symptoms at all. You might have one right now and not know it. That’s why regular dental checkups matter: dentists use X-rays and visual exams to catch decay you can’t feel. Bitewing X-rays remain the standard tool for finding cavities between teeth, though some offices now use laser fluorescence devices that measure bacterial activity on tooth surfaces.
Once decay passes through the enamel and reaches the softer layer underneath called dentin, you’ll likely start noticing sensitivity. This is the sharp, sudden discomfort you feel when you bite into something hot, cold, or sweet. Dentin contains tiny channels that connect to the nerve inside your tooth, so exposing it essentially gives stimuli a direct path inward. The pain is usually brief and goes away once the trigger is removed.
If the cavity keeps growing and reaches the innermost part of the tooth, the pulp, the pain changes character. The pulp contains the tooth’s nerve and blood supply. When bacteria invade this tissue, it becomes inflamed, a condition called pulpitis. At first, this inflammation may still be reversible if a dentist can remove the decay and seal the tooth. But once the damage progresses, the inflammation becomes irreversible, meaning the pulp can no longer heal itself. At this stage you may experience throbbing, persistent pain that doesn’t need a trigger to flare up, sometimes waking you at night.
How Fast Cavities Progress
Cavities don’t form overnight. Research shows it takes a relatively long time for decay in the enamel to penetrate into the dentin layer, with studies requiring a minimum follow-up of three years just to observe meaningful progression. That said, the speed varies widely. Factors like diet, oral hygiene, saliva flow, and whether you use fluoride all influence how quickly or slowly a cavity grows. Someone who drinks sugary beverages throughout the day will see faster progression than someone who limits sugar to mealtimes.
The dentin layer is softer and less mineral-dense than enamel, so once decay breaks through to it, the process tends to accelerate. This is one reason dentists recommend treating cavities sooner rather than later. A small filling now can prevent a much larger problem later.
What Happens If You Don’t Treat It
Left alone, a cavity follows a predictable path: enamel, then dentin, then the pulp. Once bacteria reach the pulp and the tissue can’t recover, the pulp dies. A dead tooth doesn’t always hurt immediately. In fact, the pain may temporarily stop because the nerve is no longer alive to send signals. But the infection doesn’t stop. Bacteria continue to multiply and can spread beyond the tooth’s root into the surrounding jawbone.
This often leads to a dental abscess, a pocket of pus that forms at the root tip. Abscesses cause intense, radiating pain, facial swelling, and sometimes fever. At this point the infection can spread to other areas of the head and neck. In rare but serious cases, untreated dental infections have led to deep neck infections, sepsis, and even heart valve infections. A case report in The American Journal of Case Reports documented an untreated cavity that eventually caused an abscess extending into the space around the spinal cord. These extreme outcomes are uncommon, but they illustrate why ignoring a cavity indefinitely carries real risk.
How Cavities Are Treated
The treatment your dentist recommends depends on how much tooth structure the cavity has destroyed. For small to moderate cavities confined to the enamel and outer dentin, a standard filling is the typical fix. The dentist removes the decayed material and fills the space with a composite resin or similar material. This is usually done in a single visit with local anesthesia, and you can eat normally within a few hours.
When a cavity has destroyed a larger portion of the tooth, a filling alone may not provide enough structural support. In these cases, a crown, which is a cap that covers the entire visible portion of the tooth, is often recommended. Crowns require more time and cost more than fillings, but they protect what’s left of the tooth from cracking under the force of chewing.
If decay has reached the pulp and caused irreversible damage, a root canal becomes necessary. During this procedure, the dead or infected pulp tissue is removed from the inside of the tooth, and the hollow space is sealed. The tooth itself stays in place but no longer has a living nerve. Most root canal teeth need a crown afterward because removing the pulp and surrounding decay leaves the tooth more fragile. If the tooth is too damaged to save at all, extraction is the final option.
Why Some People Get More Cavities
Cavity risk isn’t just about brushing habits. The composition and flow rate of your saliva play a significant role, since saliva neutralizes acid and delivers minerals back to your teeth. People with dry mouth, whether from medications, medical conditions, or simply genetics, are at higher risk. The specific bacteria living in your mouth matter too. Some people harbor higher levels of the acid-producing bacteria most responsible for decay.
Diet patterns are often more important than the total amount of sugar consumed. Sipping a sugary coffee over two hours creates a longer acid exposure than drinking it in ten minutes. Sticky carbohydrates that cling to tooth surfaces, like dried fruit or crackers, can be just as problematic as candy. Frequent snacking without brushing keeps the mouth in a constant acidic state, giving your saliva no chance to do its repair work.