Cavities show up as visible changes on your teeth, new sensitivity, or persistent pain, sometimes all three at once. Nearly 21% of adults between 20 and 64 have at least one untreated cavity right now, making this one of the most common health problems people walk around with unaware. The good news is that decay follows a predictable pattern, and knowing what to look for at each stage can help you catch it early.
What a Cavity Looks Like
The earliest sign of a forming cavity is a white spot on the tooth surface. This chalky, matte patch means minerals are leaching out of the enamel. At this stage, there’s no hole yet, and the process can actually be reversed with fluoride and improved oral care.
If that white spot goes untreated, it darkens to brown as the enamel continues to break down. Eventually, small pits or holes form in the tooth surface. These are true cavities. As decay advances further, spots turn black or gray, and the holes become larger and easier to see, sometimes catching food or feeling rough when you run your tongue over them.
Not every dark spot is a cavity, though. The deep grooves on your molars can hold staining from coffee, tea, or red wine without any actual decay underneath. Here’s the key difference: stains tend to affect the whole tooth or show up across multiple teeth in a similar pattern, and they can fade or change over time. A cavity is usually a single, localized spot that only gets darker and larger. If you see a visible hole, that’s always decay. Stains don’t create holes.
How a Cavity Feels
Many early cavities cause no pain at all, which is part of why they go undetected. But as decay eats through the hard outer enamel and reaches the softer layer underneath (called dentin), you’ll start to notice things.
The most common early sensation is sensitivity. Cold drinks, hot food, or anything sweet triggers a sharp, fleeting zing in one specific tooth. This happens because the protective enamel has worn thin enough to expose the more sensitive inner structure. The pain is usually mild at first and comes and goes, but it becomes more constant as the cavity grows.
Once decay reaches the innermost part of the tooth, where the nerve and blood supply live, the pain changes character. The tissue swells inside the rigid tooth, pressing on nerves with nowhere to expand. This produces a deep, throbbing ache that can radiate into your jaw and ear. At this stage, you may also notice pain when you bite down or chew. If an infection forms at the root, it can cause facial swelling, fever, and swollen glands in your neck.
Cavities You Can’t See
Some of the trickiest cavities form between teeth, where you can’t spot them in the mirror. These “interproximal” cavities account for a significant share of decay, and they announce themselves through subtler clues.
One early sign is food repeatedly getting stuck between the same two teeth. Decay erodes the contact point between teeth, creating a small gap or pocket that traps food. You might also notice a faint shadow or dark discoloration between teeth when you look closely during brushing. Persistent bad breath or a sour taste that lingers even after brushing can signal bacteria trapped in a decaying spot between teeth.
Sensitivity that seems to come from between two teeth rather than from the biting surface is another red flag, especially if cold, hot, or sweet triggers it in a specific area. A mild but persistent ache when biting down, localized to one spot, points in the same direction.
The Five Stages of Decay
Cavities don’t appear overnight. They progress through distinct stages, and recognizing where you are in that progression helps you understand what’s happening and how urgent the situation is.
Stage 1: Mineral loss. White spots appear. No structural damage yet. This is the only stage where the process is fully reversible without a filling.
Stage 2: Enamel breakdown. White spots darken to brown. Small holes begin forming in the enamel. You likely won’t feel pain yet because enamel has no nerve endings.
Stage 3: Dentin decay. Once past the enamel, decay moves faster through the softer dentin layer. Sensitivity to temperature and sweets kicks in. This is the stage where most people first realize something is wrong.
Stage 4: Pulp involvement. Decay reaches the nerve. Pain becomes significant and harder to ignore. Treatment at this point typically requires more than a simple filling.
Stage 5: Abscess. Infection at the tooth root causes severe, radiating pain, possible swelling in the gums or face, and fever. This is a dental emergency.
Stain or Cavity: How to Tell the Difference
This is the question most people are really asking when they spot something on a tooth. A few guidelines can help you sort it out before your dental visit.
- Location matters. A single dark spot on one tooth is more likely a cavity. Staining typically appears across multiple teeth, especially along the gumline or on the back surfaces where coffee and tea make the most contact.
- Persistence matters. Stains can lighten after a cleaning or with whitening toothpaste. A cavity only gets worse over time, never lighter.
- Texture matters. Run your tongue over the spot. If it feels rough, sticky, or like a pit, that suggests decay. Stains sit on a smooth surface.
- Symptoms matter. Stains don’t cause sensitivity or pain. If the dark spot comes with a zing when you drink cold water or a dull ache when chewing, it’s very likely a cavity.
What Your Dentist Can Detect That You Can’t
Self-examination has real limits. You can spot surface cavities on the front or top of your teeth, but you simply cannot see between teeth, under old fillings, or below the gumline. These are some of the most common locations for decay to hide.
Standard dental X-rays reveal cavities between teeth as dark shadows in the enamel or dentin. They’re the primary tool for catching decay that hasn’t surfaced visually yet. Some dental offices also use laser fluorescence devices, like the DIAGNOdent system, which scan tooth surfaces and measure changes in mineral density. These tools detect early demineralization that’s invisible to the naked eye. Dentists combine these technologies with a visual exam and probing to confirm whether a suspicious spot is active decay or just a deep groove.
There are no reliable at-home cavity detection kits on the market. Self-diagnosis tools for oral conditions generally lack the peer-reviewed evidence needed to demonstrate diagnostic accuracy across the range of conditions that can mimic each other in the mouth. A dark spot could be a stain, a cavity, or simply a deep groove in the enamel, and distinguishing between them often requires X-rays or specialized instruments. What you can do at home is monitor for the signs described above and use that information to schedule a visit sooner rather than later when something looks or feels off.