Cavities and Tooth Decay: Causes, Signs and Treatment
Written by NoFrills Dental clinical team
Updated 03/08/2026
A cavity is a hole in the tooth caused by acid from plaque bacteria feeding on sugar. Caught early enough it can be halted or even reversed with fluoride. Once the enamel surface breaks, it needs a filling, because a tooth cannot repair a hole on its own.
- How often you eat sugar matters more than how much you eat at once.
- Early decay, before the surface breaks, can be remineralised with fluoride.
- Decay is painless until it is deep. Pain is a late signal, not an early one.
What a cavity actually is
Your teeth are coated in plaque, a soft film of bacteria that re-forms within hours of brushing. When sugars and refined carbohydrates reach that plaque, the bacteria convert them into acid. The acid pulls minerals out of the enamel, and each time it happens the enamel is left slightly weaker. Saliva puts minerals back between attacks. Decay happens when the losses outpace the repairs, month after month.
The result is dental caries, which most people call tooth decay or a cavity. It is one of the most common chronic diseases in the world, and in Singapore roughly one in three adults surveyed had untreated decay at the time of the last national oral health survey.
What causes tooth decay
Four things have to line up: a tooth, plaque bacteria, fermentable sugar and time. Remove any one of them and decay does not progress.
- Frequency of sugar. Sipping a sweet drink over an afternoon causes far more damage than the same drink finished in ten minutes, because the mouth never returns to neutral.
- Plaque left undisturbed, especially between the teeth and along the gum line, which is why cleaning between the teeth matters as much as brushing.
- Not enough fluoride. Fluoride makes enamel more resistant to acid and helps rebuild early damage.
- Dry mouth. Saliva neutralises acid and carries minerals. Many common medicines reduce it, as do some medical conditions, and older adults are affected more often.
- Deep grooves and crowded teeth, which trap plaque where a brush cannot follow.
Decay is not contagious in the ordinary sense, but the bacteria involved can be passed between people, which is why sharing spoons with a baby or cleaning a dropped pacifier in your own mouth is discouraged.
Who is most at risk
Everyone gets some risk, but it stacks up for people who graze on sweet food and drinks through the day, people with dry mouth from medication, people wearing braces, people with receding gums where the softer root surface is exposed, and anyone who has already had several fillings. Past decay is the single best predictor of future decay.
Older adults are a particular case. Root surfaces decay faster than enamel, dry mouth is common with age and medication, and partial dentures can trap plaque against the remaining teeth.
Signs and symptoms
Early decay has no symptoms at all. That is the whole problem with waiting for pain.
- A white chalky patch on the enamel, which is demineralisation before a hole forms
- A brown or black spot or a visible hole in the tooth
- Sensitivity to sweet, cold or hot food and drink
- Food consistently packing into the same spot
- A rough edge you keep catching with your tongue
- Toothache, which usually means the decay has reached the nerve
- Bad breath or a bad taste that does not clear
Decay between teeth is invisible from the outside. It is usually found on a bitewing x-ray at a routine check-up, often years before it would have hurt.
A small filling now is quicker, cheaper and less invasive than a root canal later.
The stages, and which can be reversed
| Stage | What can be done |
|---|---|
| Early demineralisation (white spot, surface intact) | Reversible. Fluoride, better cleaning and less frequent sugar can rebuild the enamel. |
| Enamel cavity (surface has broken) | Not reversible. Needs a filling, usually small. |
| Dentine decay (past the enamel) | Larger filling. Sensitivity is common at this point. |
| Decay reaching the pulp (nerve) | Root canal treatment, usually followed by a crown. |
| Abscess or unrestorable tooth | Root canal or extraction, then replacement of the tooth. |
The whole argument for regular check-ups sits in the first two rows of that table. The gap between a reversible white spot and a tooth needing a crown is measured in months to a few years, and none of it hurts along the way.
How cavities are treated
For early damage, we use high strength fluoride varnish, advice on cleaning, and a review to see whether it has stabilised. For a cavity that has broken through, the decayed tissue is removed and the tooth is rebuilt with a filling, most often tooth coloured composite. Larger cavities may need an inlay or a crown to hold the remaining tooth together. When decay has reached the nerve, root canal treatment saves the tooth by removing the infected pulp and sealing the space.
On cost: fillings, extractions, root canal treatment and crowns are all claimable under various Singapore schemes depending on your eligibility. Bring your card, and see our guides to CHAS subsidies in 2026 and using MediSave for dental treatment.
How to prevent decay
- Brush twice daily with a fluoride toothpaste of at least 1,000 ppm, and spit without rinsing so the fluoride stays on the teeth. Our oral hygiene guide covers the details.
- Clean between your teeth once a day. Most adult decay starts in the spaces a brush cannot reach, and a routine scaling and polishing every six months clears the hardened deposits that daily cleaning leaves behind.
- Cut the number of sugar occasions, not just the amount. Keep sweet things to mealtimes, and drink water in between.
- Ask about fluoride varnish or a higher strength toothpaste if you have had several fillings, dry mouth, or receding gums.
- Have a check-up at the interval we recommend, at any of our clinics: Suntec City, Marina Square, Marine Parade or Tanjong Katong.
Think you might have a cavity?
Sensitivity, a rough edge or food packing in one spot is worth ten minutes of our time. Small problems stay small when they are found early.
Frequently asked questions
What is a cavity?
A cavity is a hole in a tooth caused by dental caries. Bacteria in plaque turn sugars into acid, the acid dissolves minerals out of the enamel, and once enough mineral is lost the surface collapses into a hole. Before that point the damage can often be reversed with fluoride, but once the surface has broken the tooth cannot repair itself and needs a filling.
Can a cavity heal on its own?
Only at the earliest stage. A white spot lesion, where minerals have been lost but the enamel surface is still intact, can be remineralised with fluoride, better cleaning and less frequent sugar. Once there is an actual hole, it will not heal and it will keep getting bigger until it is restored.
Does a cavity always hurt?
No, and this is the main reason people are surprised at check-ups. Enamel has no nerve supply, so early and moderate decay is completely painless. Pain usually means the decay has reached the dentine or the nerve, at which point treatment is more involved. Decay between teeth is often found only on x-rays.
How long does it take for a cavity to form?
Usually months to years rather than days, depending on how often sugar is consumed, how well the area is cleaned, how much fluoride the tooth gets and how much saliva is present. Decay can move much faster on exposed root surfaces, in a dry mouth, or around braces.
Is tooth decay contagious?
The disease is not passed on like a cold, but the bacteria involved can be transferred through saliva, for example by sharing spoons or cleaning a dropped pacifier in your own mouth. Whether decay develops still depends on diet, cleaning and fluoride.
Can I just have the tooth taken out instead of filled?
It is almost always better to keep a tooth that can be restored. Losing a tooth changes how you chew, allows the neighbouring teeth to drift, and leads to bone loss in that area over time, which then makes replacement more complicated and more expensive. Extraction is the right answer when a tooth is beyond repair, not as a shortcut.
Related reading
More in this series: bad breath, daily oral hygiene, flossing and interdental cleaning, mouth spots and sores and TMJ and jaw pain.
Sources: World Health Organization on sugars and dental caries, Cochrane review of fluoride toothpaste concentrations (2019), National Dental Centre Singapore on the National Adult Oral Health Survey.