Dental cavities form when acids produced by bacteria in your mouth dissolve the mineral structure of your teeth. It’s the most common chronic disease worldwide, and CDC data from 2024 shows that nearly 21% of American adults aged 20 to 64 have at least one untreated cavity right now. The process behind every cavity is the same: bacteria feed on sugars, produce acid, and that acid eats away at tooth enamel faster than your body can repair it.
How a Cavity Actually Forms
Your teeth are covered in a sticky film of bacteria called plaque. When you eat or drink anything containing fermentable carbohydrates (sugars and starches), certain bacteria in that plaque break those carbohydrates down and produce organic acids as a byproduct. The most damaging of these is lactic acid.
Tooth enamel is made of a crystalline mineral called hydroxyapatite, which is rich in calcium and phosphate. Under normal conditions, this mineral is incredibly hard. But when bacterial acids drop the pH at the tooth surface below about 5.5, enamel starts to dissolve. Calcium and phosphate ions leach out of the tooth surface and subsurface, creating what dentists call a “white spot lesion,” the earliest visible sign of decay. At this stage, the damage is still reversible. Left unchecked, the mineral loss deepens, the enamel breaks down completely, and a full cavity forms.
This isn’t a one-time event. Your mouth cycles between acid attacks and recovery dozens of times a day. Every time you eat, pH drops. Between meals, saliva gradually neutralizes the acid and delivers calcium and phosphate back to the tooth surface. A cavity develops when the balance tips toward destruction: too many acid attacks, not enough recovery time, or both.
The Bacteria Behind the Damage
Not all mouth bacteria cause cavities. The ones that do share two key traits: they’re highly efficient at converting sugars into acid, and they can survive in the acidic environment they create. The most well-known cavity-causing species is Streptococcus mutans, but it doesn’t work alone. Lactobacillus species and bifidobacteria also play major roles, particularly as cavities progress deeper into the tooth.
Lactobacilli are especially effective at driving decay because they carry an unusually broad set of enzymes for breaking down different types of carbohydrates, not just simple sugars but also starches and complex oligosaccharides. They rapidly convert these into lactic acid and other acidic byproducts. What makes them particularly persistent is their ability to tolerate the very acid they produce. They use multiple survival mechanisms, including proton pumps and ammonia-producing pathways, to keep their internal pH stable even as the environment around them becomes hostile to other bacteria. This means they can keep producing acid long after other species have been suppressed.
Sugar, Starch, and Frequency
Sugar gets most of the blame for cavities, and sucrose (table sugar) is indeed the most cavity-promoting carbohydrate. But the picture is more nuanced than “sugar causes cavities.” Starches, fruit sugars, and even the natural sugars in milk can all serve as fuel for acid-producing bacteria. What matters most is not just how much sugar you eat, but how often and for how long your teeth are exposed to it.
Sipping a sugary drink over two hours creates a sustained acid attack that’s far more damaging than drinking the same amount in five minutes. Sticky foods that cling to tooth surfaces, like dried fruit, caramel, or crackers that pack into grooves, extend the duration of acid exposure. Each new exposure restarts the pH drop, giving your saliva less time to neutralize the acid and begin repairs.
How Saliva Protects Your Teeth
Saliva is your mouth’s primary defense against cavities. It works in several ways at once: it physically washes food particles and bacteria off tooth surfaces, it contains bicarbonate and phosphate buffers that neutralize bacterial acids, and it acts as a mineral reservoir, carrying dissolved calcium and phosphate ions that can redeposit into weakened enamel. Saliva also contains compounds like urea and sialin (a peptide rich in the amino acids arginine and lysine) that help raise pH after an acid attack.
This is why anything that reduces saliva flow dramatically increases cavity risk. Dry mouth, known clinically as xerostomia, removes the mouth’s main protective mechanism. According to the American Dental Association, the most frequent cause of reduced saliva flow is medication use. Over 100 medications have moderate to strong evidence of causing salivary gland dysfunction, spanning common drug categories: antihistamines, antidepressants, blood pressure medications, decongestants, diuretics, muscle relaxants, pain medications, and even newer GLP-1 receptor agonists used for diabetes and weight loss. Medications with anticholinergic effects, including tricyclic antidepressants and antihistamines, are the most common culprits.
If you take any of these and notice persistent dry mouth, that’s a direct cavity risk factor worth addressing with your dentist.
Why Some People Get More Cavities
You’ve probably noticed that some people seem to get cavities no matter what they do, while others rarely brush and never get one. Part of this comes down to genetics. Research into caries susceptibility has identified four main categories of genes that influence cavity risk: genes involved in enamel development, saliva composition, immune response, and taste perception.
One concrete example involves a gene called TUFT1, which plays a role in enamel formation. People with a specific variant (the C/C genotype at a particular location in this gene) have significantly higher rates of tooth decay. Interestingly, researchers found that this genetic link didn’t correspond to visible changes in enamel structure or mineral composition. Instead, the increased risk may come from differences in how well the enamel surface resists bacterial attachment or how effectively it remineralizes after acid exposure. A separate finding showed a statistically significant correlation between lower calcium content in enamel and higher rates of decay, suggesting that some people’s teeth are simply built with less mineral density to begin with.
Beyond genetics, other individual risk factors include the natural shape and depth of the grooves on your molars (deeper grooves trap more bacteria and are harder to clean), how much and how often you eat fermentable carbohydrates, whether you breathe through your mouth (which dries it out), and conditions like acid reflux that expose teeth to stomach acid.
How Fluoride Shifts the Balance
Fluoride is the single most effective tool for preventing cavities, and its mechanism explains why. When fluoride is present during the remineralization process, it gets incorporated into the tooth’s crystal structure. Normally, tooth enamel is made of hydroxyapatite. When fluoride ions replace the hydroxyl groups in that crystal lattice, the result is fluorapatite, a mineral that is structurally more stable and more resistant to acid dissolution. In practical terms, fluorapatite starts dissolving at a lower pH than regular enamel, meaning your teeth can withstand more acid before mineral loss begins.
Fluoride also promotes the redeposit of calcium and phosphate from saliva back into damaged enamel, accelerating repair. This is why fluoride toothpaste and fluoridated water are effective even after teeth have already formed. The benefit isn’t just about building harder teeth during childhood. It’s about tipping the daily balance between mineral loss and mineral repair in favor of repair, every time you brush.
Cavities at Different Ages
Cavity patterns shift across the lifespan. About 11% of children aged 2 to 5 already have untreated decay in their baby teeth, and that number climbs to nearly 18% among children aged 6 to 8. Among adolescents aged 12 to 19, roughly 10% have untreated decay in their permanent teeth. The rate peaks in working-age adults at 21%, then drops to about 13% in adults over 65, likely reflecting both tooth loss and generational differences in dental care access.
Children are particularly vulnerable because newly erupted teeth have enamel that hasn’t fully matured and is more porous. Older adults face a different challenge: gum recession exposes tooth roots, which are covered in cementum and dentin rather than enamel. Dentin begins dissolving at a higher pH than enamel (around 6.0 to 6.5 rather than 5.5), making root surfaces significantly more susceptible to decay. Combined with the dry mouth effects of medications that become more common with age, this creates a perfect environment for cavities to develop on surfaces that were protected for decades.