Rotten teeth are caused by acid. Bacteria in your mouth feed on sugars from food and drink, producing acid that dissolves the hard mineral surface of your teeth. When this process outpaces your mouth’s natural ability to repair itself, the result is decay: soft spots, cavities, and eventually the deep, dark damage most people picture when they think of rotten teeth. About 21% of American adults between 20 and 64 have at least one tooth with untreated decay, according to the CDC’s 2024 oral health report.
How Bacteria Turn Sugar Into Acid
Your mouth is home to hundreds of species of bacteria, but one in particular drives the decay process. When you eat something sugary or starchy, bacteria metabolize those simple sugars and produce lactic acid as a byproduct. In the presence of excess glucose, lactic acid production dominates, creating a concentrated acidic environment right on the tooth surface.
Tooth enamel, the hard outer shell of your teeth, starts dissolving when the pH in your mouth drops below roughly 5.5. For reference, that’s less acidic than a lemon but far more acidic than your saliva normally allows. The layer underneath the enamel, called dentin, is even more vulnerable and begins breaking down at a pH around 6.0. Every time you eat or drink something containing sugar, this acid attack begins within minutes and can last for 20 to 30 minutes afterward.
What Happens as Decay Progresses
Tooth decay doesn’t appear overnight. It moves through distinct stages, and catching it early makes a significant difference in what treatment looks like.
The first visible sign is a white spot on the tooth surface. This is a patch of demineralization where minerals have started leaching out of the enamel, but the surface itself is still intact. At this stage, the damage is reversible. If the acid attacks keep coming, though, the smooth enamel surface becomes rough and porous. Small micro-cavities form, and eventually a full cavity breaks through.
Once decay penetrates the enamel, it reaches the dentin, a softer, yellowish layer that makes up most of the tooth’s structure. Dentin decays faster than enamel because it’s less mineralized. The tooth may respond by producing a defensive layer of new dentin to shield the innermost part, the pulp, which contains nerves and blood vessels. Clinically, decayed dentin feels leathery and soft. At this point you might notice sensitivity to hot, cold, or sweet foods.
If bacteria reach the pulp, the infection can spread beyond the tooth itself, leading to an abscess (a pocket of pus at the root tip), infection of the surrounding bone, or in serious cases, spreading infection into the jaw or soft tissues of the face. This is the stage where a tooth looks and feels truly “rotten,” often darkened, broken down, and painful.
Sugar: Amount Matters More Than Frequency
For decades, dentists emphasized that how often you eat sugar matters more than how much. Recent research tells a more nuanced story. A large study of U.S. adults found that the total amount of added sugar consumed was more consistently and strongly associated with decay than the number of times per day sugar was eaten. Each increase of about 55 grams of added sugar per day (roughly the amount in a 16-ounce soda) was linked to an 11% higher decay score. Frequency of sugar intake had a much smaller, and sometimes statistically insignificant, association once total amount was accounted for.
This doesn’t mean frequency is irrelevant. Sipping a sugary drink over several hours keeps your mouth acidic for longer stretches than drinking it quickly. But the takeaway is that reducing overall sugar intake is likely more protective than simply consolidating your sugar into fewer sittings.
Dry Mouth and Medications
Saliva is your mouth’s primary defense against decay. It constantly bathes your teeth, washing away food debris and bacteria. It acts as a buffer, neutralizing acids before they can do serious damage. And it serves as a mineral reservoir, supplying calcium, phosphate, and fluoride that help rebuild early enamel damage.
When saliva production drops, all of these protective functions weaken. Dry mouth is one of the most common and underappreciated causes of rapid tooth decay in adults. A systematic review identified 22 drug categories with strong evidence of reducing salivary flow. These include many widely prescribed medications: certain blood pressure drugs (beta-blockers, calcium channel blockers, diuretics), antidepressants (both older tricyclics and newer SSRIs like fluoxetine, sertraline, and paroxetine), overactive bladder medications, anti-seizure drugs, antihistamines, and some anti-nausea medications. Anti-muscarinic drugs are particularly well-known culprits because they block the nerve signals that tell salivary glands to produce saliva.
If you take one or more of these medications and notice your mouth feels dry, that dryness is likely accelerating decay even if your diet and brushing habits haven’t changed.
Acid Reflux and Stomach Acid
Bacteria aren’t the only source of acid that destroys teeth. Gastroesophageal reflux disease (GERD) brings stomach acid into the mouth, and stomach acid has a pH below 2.0, far more corrosive than anything oral bacteria produce. This type of damage is called dental erosion rather than dental caries, because it’s a direct chemical attack without bacteria involved.
The pattern of damage from reflux is distinctive. Stomach acid hits the back surfaces of the upper front teeth first, since those are closest to the throat and least protected by saliva glands. If reflux continues over time, the chewing surfaces of the back teeth erode next. The lower teeth are initially shielded by the tongue but eventually affected too. People with chronic reflux, bulimia, or frequent vomiting often show widespread erosion that looks very different from the cavity-style decay caused by bacteria.
Baby Bottle Tooth Decay
Young children are especially vulnerable to a specific pattern of decay. About 11% of children aged 2 to 5 have untreated decay in their baby teeth, and the number climbs to nearly 18% for children aged 6 to 8. One of the primary drivers is prolonged exposure to sugary liquids, particularly when a baby falls asleep with a bottle of milk, formula, or juice. The liquid pools around the upper front teeth while saliva flow drops during sleep, creating ideal conditions for acid damage.
Cavity-causing bacteria aren’t present in a baby’s mouth at birth. They’re typically transmitted from a caregiver, often through shared utensils or cleaning a pacifier with your mouth. Once those bacteria colonize the child’s mouth, they stay. Dipping pacifiers in sugar or honey, a practice that still occurs, directly feeds those bacteria.
How Your Teeth Fight Back
Decay isn’t a one-way street. Your teeth are constantly losing and regaining minerals in a process called remineralization. Saliva supplies calcium and phosphate ions that redeposit onto weakened enamel surfaces. Fluoride accelerates this process and makes the repaired enamel harder than the original. When fluoride is incorporated during remineralization, it forms a crystal structure called fluorapatite, which is more resistant to future acid attacks than the hydroxyapatite that enamel is naturally made of.
This is why fluoride toothpaste and fluoridated water are effective at preventing decay. They don’t just coat your teeth temporarily. They change the chemistry of the repair process, producing a tougher surface each time your enamel rebuilds. For the same reason, children who don’t get adequate fluoride exposure have a higher risk of decay from the start.
The balance between acid damage and mineral repair determines whether your teeth stay healthy or rot. Anything that tips that balance, more sugar, less saliva, chronic acid exposure, or inadequate fluoride, pushes your teeth toward decay. Understanding that balance is the key to understanding why teeth rot and what you can actually control about it.