Tooth Decay: How Cavities Form, Progress and Are Diagnosed
Tooth decay (dental caries) is damage caused when plaque bacteria turn sugars into acids that dissolve tooth enamel. For adults, this page explains how cavities form, the stages from white spot to pulp, why decay does not always hurt and when to see a dentist, how it is diagnosed and when it can be reversed. The core point is that early decay can be reversed, so catching it early matters.
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Clinically reviewed by Dr Ibraheem Ijaz, dentist · GDC 301711
Medical disclaimer: This page is for general information only. It does not replace an examination or treatment by a dentist.
Dr Ibraheem Ijaz, GDC 301711, principal dentist at Deepcar Dental Care, reviewed this page. He is a General Dental Practitioner with a BDS, BSc (Hons), MSc (Leeds) and postgraduate qualifications in implant, restorative and aesthetic dentistry.
What is tooth decay?
Tooth decay, or dental caries, is a disease in which bacterial acids gradually dissolve the minerals in a tooth. Delivering Better Oral Health, the UK government's prevention toolkit, says micro-organisms in plaque, the biofilm on teeth, break down sugars into acids that demineralise enamel.
A 2024 review of the caries microbiome names Streptococcus mutans as one of several acid-producing bacteria involved. Plaque itself is explained in plaque and calculus, and decay in baby teeth in tooth decay in children.
How do cavities form?
Cavities form when acid attacks remove minerals from enamel faster than they can be replaced. Delivering Better Oral Health calls this cycle demineralisation and remineralisation.
Within one minute of having sugar, plaque becomes more acidic as bacteria turn the sugar into acid. It slowly recovers over 20 to 40 minutes, helped by saliva and as the sugar is used up. Minerals leave the enamel while plaque is acidic and can return when sugar intakes are spaced some hours apart, more effectively when fluoride is present.
How often you have sugar matters as well as how much, as the guide to diet and tooth decay explains.
What are the stages of tooth decay?
Untreated, tooth decay can spread from an early enamel change to a cavity and then into the pulp at the centre of the tooth. The table draws on NHS guidance and the International Caries Classification and Management System (ICCMS). The layers, enamel, dentine and pulp, have their own guide.
| Stage | What is happening | What guidance describes |
|---|---|---|
| Initial (white spot lesion) | A white band along the gum line on a smooth surface, or a light or dark brown area in a groove. The surface is unbroken. | Non-surgical care such as fluoride applications, sealants and better oral hygiene (ICCMS). The NHS says a dentist can prescribe fluoride treatments like mouthwash or varnish to reverse early decay. |
| Moderate | The enamel starts to break down, or a dark shadow shows through from the dentine. | Minimally invasive care with no or minimal removal of dentine, depending on the stage and whether the decay is active (ICCMS). |
| Extensive (cavity) | A clear hole exposes the dentine. | The NHS says you will probably need a filling. |
| Pulp | Decay has reached the soft tissue in the middle of the tooth. | The NHS says you may need root canal treatment; sometimes the tooth may need to be removed. |
If you think you have tooth decay, the NHS advises seeing a dentist as soon as possible. A hole in a tooth explains what a visible hole means, and cavity repair is covered under white fillings.
To ask about tooth decay, book online or call 0114 288 2121.
Why does tooth decay not always hurt, and when should you see a dentist?
Tooth decay may not hurt at first because toothache comes from inflammation of the pulp inside the tooth, which early decay does not yet affect. The NHS notes that decay may not cause any symptoms at first and advises regular check-ups even without problems.
A 2011 review of dental pain explains the link. Dentine, the layer under the enamel, is made of tiny fluid-filled tubules that connect to nerve endings in the pulp, and bacterial products from decay can reach the pulp through them. The review describes the inflammation as reversible at first, with short, sharp pain to cold and sweet things, and irreversible if the cause persists, with prolonged pain to heat.
See a dentist as soon as possible if you think you have tooth decay, the NHS advises. Its tooth decay page adds problems such as toothache or a sharp pain in your tooth when eating or drinking, and gums that are painful, swollen or bleed when you brush.
For toothache, the NHS says to see a dentist if it lasts more than 2 days, does not go away with painkillers, comes with a high temperature, pain when you bite, red gums or a bad taste in your mouth, or comes with a swollen cheek or jaw. If you do not have a dentist or cannot get an emergency appointment, call 111 or visit 111 online.
Go to A&E if you have toothache and the area around your eye or your neck is swollen, or swelling in your mouth or neck makes it difficult to breathe, swallow or speak. Do not drive. Ask someone to drive you or call 999 for an ambulance.
How is tooth decay diagnosed?
Tooth decay is diagnosed by examining each tooth surface and, where needed, taking X-rays to see how far it has spread.
In general, under the ICCMS caries management system, the earliest enamel changes may only show once the surface is dried thoroughly with air, and the dentist judges whether decay is active or arrested. X-rays help estimate how far decay has spread, though depth does not always match whether a hole has formed, especially between teeth, and two X-rays taken some time apart are normally needed to show progression. The College of General Dentistry's Selection Criteria for Dental Radiography guides their use.
Can tooth decay be reversed?
Early tooth decay can be reversed, because early enamel lesions can remineralise, Delivering Better Oral Health explains. Once a hole has formed, the NHS says a filling is probably needed.
The guidance describes two levers: cutting what drives demineralisation, notably sugar, and boosting what favours remineralisation, particularly fluoride and mineral ions; how fluoride works has its own guide. ICCMS treats active lesions as more likely than inactive ones to progress, arrest or regress, and a 2024 review of the caries microbiome notes that initial (white spot) lesions are clinically reversible and many will not progress to cavities.
Frequently asked questions
Is a brown or black spot on a tooth always active decay?
Not always. Some white or brown marks are not decay, and the ICCMS caries system describes arrested decay as whitish, brownish or black, hard and smooth, and possibly shiny. Active early decay tends to look whitish or yellowish, dull and rough, where plaque collects. The NHS advises seeing a dentist as soon as possible if you think you have decay.
Can adults get new tooth decay?
Yes. Delivering Better Oral Health says new decay can develop at any age. Decay on root surfaces becomes more common with age, and exposed roots, a dry mouth, poor plaque control and past decay on the crowns of teeth are signs of a higher risk of root decay.
Can medicines raise the risk of tooth decay?
Some can. Delivering Better Oral Health says liquid, chewable or soluble medicines containing sugar can add to how often, and possibly how much, sugar you take in. Certain medicines also reduce saliva flow, which raises decay risk. The NHS advises using sugar-free medicines. If you take a medicine long term, mention it at your check-up.
Does chewing sugar-free gum help prevent tooth decay?
It might, but the evidence is tentative. Delivering Better Oral Health cites a systematic review suggesting that chewing sugar-free gum reduces new decay compared with not chewing, with considerable variation and trials of generally moderate quality. Brushing twice a day with fluoride toothpaste remains a strongly recommended step.
How often should you have a check-up to catch decay early?
It depends on your risk. Delivering Better Oral Health advises adult recall intervals of 3 to 24 months, based on oral health needs and disease risk, and notes no evidence that a specific interval changes how decay develops or progresses. At Deepcar Dental Care, the standard recall interval for a healthy adult is 6 months.
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