What causes cavities? The short answer
Cavities are caused by bacteria in your mouth that turn sugar into acid. These bacteria live in plaque, a sticky, colorless film that forms on your teeth.1 Whenever you eat or drink something with sugar or starch, they use it to make acid, and the acid starts to dissolve minerals from enamel, the hard outer layer of the tooth.17
Your saliva and fluoride fight back by putting minerals back in. It's a daily tug of war. If acid wins too often, a weak white spot turns into a real hole: a cavity.12
So sugar doesn't rot teeth by itself, and nobody "drills" the hole. Living things in your own mouth dig it, one splash of acid at a time.
How do cavities form, step by step?
- Bacteria settle in. Your mouth is home to hundreds of kinds of bacteria. Most are harmless or helpful, but a few, especially Streptococcus mutans, play a big part in tooth decay.111
- Plaque builds up. Bacteria, food bits and saliva form a sticky film. Plaque starts building up within about 20 minutes after eating, and it's most common on the back teeth, near the gum line and around the edges of fillings.9
- The bacteria feast. They feed on sugars and starches, from candy, soda and juice to bread, crackers and milk.113 S. mutans even splits table sugar and uses part of it to build a sticky coat that glues it to the tooth.12 (Splitting sugar is a job for enzymes; see how enzymes work.)
- They release acid. Like every living thing, bacteria make waste, and theirs is acid. The sticky plaque holds that acid right against the tooth.7
- Enamel loses minerals. The acid dissolves calcium and phosphate minerals from the enamel. This is called demineralization.1
- Saliva and fluoride repair it. Minerals from saliva, plus fluoride, rebuild the enamel between acid attacks. This is remineralization.1
- If acid keeps winning, a chalky white spot appears, then the surface breaks down into a hole.2
Why how often you snack matters more than how much
This is the part most people miss. Every time you eat or drink something sugary or starchy, the plaque on your teeth becomes more acidic within minutes. In 1944 a scientist named Stephan measured this and drew what's now called the Stephan curve: after a sugar rinse, plaque acid rises fast, and when it goes below about pH 5.5, enamel starts to lose minerals. Then saliva slowly brings it back to normal, usually in 30 to 60 minutes.10
That means a big dessert with dinner is one acid attack. Sipping a soda for three hours, or nibbling crackers all afternoon, can be many attacks in a row, with no break for repair. That's why health experts advise limiting between-meal snacks: it "gives teeth a chance to repair themselves."1 Sticky foods are worse too, because they cling to the teeth and keep feeding the bacteria.9
You can try this yourself in lab 4 above: tap the day to add snacks and watch the minutes in the danger zone add up.
Inside a tooth: enamel, dentin and pulp
A tooth has layers. On top is enamel, the hard, shiny shell. Under it is dentin, which is softer. In the middle is the pulp, with the tooth's nerves and blood vessels, which runs down into the roots in your jaw.9
Early decay in the enamel usually doesn't hurt at all.2 As a cavity grows through the dentin toward the nerve, the tooth can ache or feel sharp twinges with sweet, hot or cold food and drinks.38 If bacteria reach the pulp, it can get infected and form an abscess, a painful pocket of pus.23 That's why dentists like to catch cavities early.
Can cavities go away or be reversed?
Early decay, yes. A real hole, no. At the white-spot stage, enamel can still repair itself using minerals from saliva and fluoride from toothpaste, tap water or a dentist's treatment. Decay can be stopped or even reversed at this point.12
Once the surface breaks down into a cavity, the damage is permanent and a dentist needs to repair it.1 A cavity that stops hurting hasn't healed; it can keep growing.3
How are cavities treated?
Dentists use numbing medicine so fillings don't hurt, and treatment is easier and cheaper when it's done early.9
What causes cavities in kids?
The same recipe as in adults: plaque bacteria plus frequent sugar. But kids get them a lot. Cavities are the most common chronic disease of childhood in the United States.4 Half of children aged 6 to 9 have had at least one cavity,3 and more than half of teens aged 12 to 19 have had a cavity in a permanent tooth.4
Most cavities in children and teens start in the molars, the back teeth, because their chewing surfaces have deep pits and grooves where food and bacteria hide from the toothbrush. The first permanent molars come in around ages 5 to 7, and the second ones around 11 to 14.1
And baby teeth matter too: cavities in them can cause pain, make eating, speaking and learning harder, and an infection can hurt the adult teeth growing underneath.3
Are cavities contagious?
Tooth decay is an infection with certain bacteria,1 and bacteria do travel between mouths. A ten-second kiss can pass around 80 million bacteria.13 But catching the bacteria isn't enough to get a cavity: they still need frequent sugar, and the acid still has to beat your saliva and fluoride.
How to prevent cavities
- Brush twice a day with fluoride toothpaste. It scrubs off plaque and leaves fluoride on your teeth. The NHS says to spit after brushing, not rinse.28
- Clean between your teeth every day with floss or small interdental brushes.27
- Drink fluoridated tap water. It cuts cavities by about 25% in children and adults.6 Most bottled water doesn't contain enough fluoride to help.1
- Snack less often. Keep sweets and sugary drinks for special occasions, and have nothing sugary after brushing at bedtime.1
- Ask about sealants. These thin plastic coatings on the back teeth can prevent up to 80% of cavities for 2 years and keep protecting against about half of cavities for up to 4 years.5
- Get regular check-ups. A dentist can remove plaque, spot early decay and add fluoride. The NHS suggests a check-up at least once a year if you're under 18.18
Common misconceptions
"Sugar rots your teeth." Not directly. Bacteria turn sugar into acid, and the acid does the damage.
1
"Only sweets cause cavities." Starches like bread, crackers and cereal feed the bacteria too.
113
"It's all about how much sugar." How often matters a lot: every snack starts a new acid attack.
110
"No pain means no cavity." Early tooth decay usually has no symptoms.
2
"Baby teeth fall out, so cavities don't matter." They can cause pain and infection that can harm the adult teeth underneath.
3
FAQ
What causes cavities in simple words?
Bacteria in the sticky plaque on your teeth eat sugars and starches and give off acid. The acid dissolves minerals from the enamel. If acid wins over saliva and fluoride often enough, the weak spot breaks down into a hole: a cavity.
Can cavities go away on their own?
An early white spot, before there is a hole, can be stopped or even reversed with fluoride and saliva. Once a real hole forms, it is permanent damage and a dentist has to fix it, usually with a filling.
Do cavities mean I have bad hygiene?
Not always. Brushing matters, but so do how often you eat or sip sugary things, how much fluoride you get, how much saliva you make and the deep grooves in your back teeth. Most people get a cavity at some point in their lives.
How do I stop getting cavities?
Brush twice a day with fluoride toothpaste, clean between your teeth every day, cut down on sugary snacks and drinks between meals, ask your dentist about sealants and go for regular check-ups.
Are cavities contagious?
The bacteria that help cause cavities can pass from one mouth to another, for example by kissing. But a cavity only forms if those bacteria also get frequent sugar and the acid beats the tooth's repair system.
What causes cavities in kids?
The same thing as in adults: plaque bacteria plus frequent sugar or starch. Kids are at extra risk from sipping juice or soda through the day, snacking often, and new molars with deep grooves. Half of US children aged 6 to 9 have had a cavity.
How are cavities treated?
A very early spot can be treated with fluoride varnish. A cavity is cleaned out and filled, often with tooth-colored composite resin. Very large cavities may need a crown, and decay that reaches the nerve may need a root canal.
Sources
- NIH / NIDCR, The Tooth Decay Process: How to Reverse It and Avoid a Cavity
- NIH / NIDCR, Tooth Decay
- CDC, About Cavities (Tooth Decay)
- CDC, Oral Health Tips for Children
- CDC, About Dental Sealants
- CDC, About Community Water Fluoridation
- American Dental Association (MouthHealthy), Tooth Decay and Plaque
- NHS, Tooth Decay
- MedlinePlus (NIH), Dental Cavities and Tooth Anatomy
- Salivary Stephan curve kinetics study (BMC Oral Health, via PubMed Central): background on the Stephan curve
- StatPearls (NCBI Bookshelf), Diet and Nutrition to Prevent Dental Problems
- Encyclopaedia Britannica, Streptococcus mutans
- University of Illinois Chicago College of Dentistry, The True Story of Why You Get Cavities