Cavities in adults develop when bacteria on your teeth produce acid that dissolves tooth enamel faster than your body can repair it. Nearly 21% of adults aged 20 to 64 have at least one untreated cavity, according to the CDC’s 2024 Oral Health Surveillance Report. While the basic process is the same one that causes childhood cavities, adults face a distinct set of risks: receding gums, aging dental work, dry mouth from medications, and decades of accumulated wear.
How Cavities Actually Form
Your mouth is home to hundreds of bacterial species, but one in particular drives most tooth decay. Streptococcus mutans thrives on sugar, metabolizing carbohydrates and producing lactic acid as a byproduct. That acid lowers the pH on the tooth surface, pulling calcium and phosphate minerals out of your enamel in a process called demineralization. When the pH drops below roughly 5.5, enamel begins dissolving.
This doesn’t happen all at once. Your saliva naturally buffers acid and redeposits minerals back onto your teeth throughout the day. A cavity only forms when the balance tips toward destruction, meaning acid attacks happen more frequently or last longer than your saliva can counteract. That’s why the causes of cavities aren’t just about bacteria. They’re about everything that shifts this balance: diet, saliva flow, gum health, and the condition of previous dental work.
Sugar Frequency Matters More Than Amount
S. mutans builds sticky, protective colonies on your teeth using sugars, particularly sucrose. Every time you eat or drink something containing sugar or refined carbohydrates, those bacteria produce a burst of acid that lasts about 20 to 30 minutes. Sipping a sugary coffee over two hours creates a much longer acid exposure than drinking it in five minutes, even though the total sugar is identical.
Starchy, sticky foods that cling to teeth between meals are equally problematic. Crackers, dried fruit, and granola bars can lodge in the grooves of molars and feed bacteria long after you’ve finished eating. The pattern of exposure, not just the volume of sugar, is what determines your risk.
Dry Mouth From Medications
Saliva is your primary natural defense against cavities. It’s 99% water, but the remaining 1% contains calcium, phosphate, bicarbonate (which neutralizes acid), antibacterial proteins, and mucins that coat and protect your teeth. When saliva flow drops, all of these protections diminish at once. Acid lingers longer, minerals aren’t replaced, and bacteria multiply more freely.
The most common cause of reduced saliva in adults is medication. Over 100 medications have strong to moderate evidence of impairing salivary gland function. The worst offenders are drugs with anticholinergic effects: tricyclic antidepressants, antihistamines, blood pressure medications, and antiseizure drugs. But the list extends to decongestants, muscle relaxants, diuretics, pain medications, and even newer GLP-1 receptor agonists used for diabetes and weight loss. If you take one or more of these daily, your cavity risk increases significantly, sometimes dramatically.
Adults who never had cavity problems in their twenties sometimes develop several at once in their forties or fifties after starting a new medication. The connection often goes unrecognized because the dry mouth can be subtle, worse at night or just mild enough to dismiss.
Gum Recession and Root Cavities
This is a cavity type that’s almost exclusive to adults. As gums recede (from aging, gum disease, or years of aggressive brushing), the root surfaces of your teeth become exposed. Root surfaces are fundamentally different from the enamel crowns of your teeth. They contain less mineral and more organic material, with smaller mineral crystals that dissolve more easily in acid.
Root cavities progress through a two-step attack. First, bacterial acid dissolves the minerals in the root surface, just like it does on enamel. But then a second process kicks in: enzymes from saliva and from the tooth structure itself begin breaking down the exposed protein matrix of the root. This double assault means root cavities can spread faster and wider than enamel cavities, often forming soft, shallow lesions along the gum line that are easy to miss visually.
Gum recession affects a significant proportion of adults and becomes more common with age. Periodontal disease, smoking, and even some dental procedures can accelerate it. If your dentist has mentioned recession, your root surfaces are at risk.
Decay Around Old Fillings
Adults who had cavities filled years ago face a risk that children simply don’t: recurrent decay around existing dental work. Fillings don’t last forever. Over time, the seal between a filling and the surrounding tooth can break down from the constant pressure of chewing and grinding. Small cracks, chips, or gaps form at the margins, creating entry points for bacteria.
Once bacteria slip beneath a filling, they’re impossible to reach with a toothbrush or floss. Decay progresses hidden under the restoration, sometimes without symptoms until it’s extensive. This is one reason adults who “already had all their cavities fixed” can still develop new ones in the same teeth. Recurrent decay is one of the most common reasons fillings need to be replaced.
Early Cavities vs. Advanced Cavities
Not every cavity needs a drill. Dentists classify decay on a spectrum from initial to extensive using visual criteria. Early-stage lesions (scores 1 to 2 on the International Caries Detection and Assessment System) appear as white or brown spots on intact enamel. At this point, the surface hasn’t broken down yet, and the process can sometimes be reversed or halted without removing any tooth structure. Options include fluoride treatments, sealants placed over the weakened area, or topical agents like silver diamine fluoride that stop bacterial activity in its tracks.
Once a cavity progresses to visible holes, rough edges, or deep damage (scores 5 to 6), the tooth structure is permanently lost and a restoration becomes necessary. The practical takeaway: catching decay early gives you more options and less invasive treatment. This is a major reason regular dental exams matter even when nothing hurts.
What You Can Do About It
Fluoride remains the single most effective tool for preventing adult cavities. Standard toothpaste contains around 1,000 to 1,500 parts per million (ppm) of fluoride. For adults at higher risk, the ADA supports prescription-strength toothpaste containing 5,000 ppm fluoride, which is more than three times the concentration of regular toothpaste. If you’ve had multiple cavities in the past few years, or you deal with chronic dry mouth, ask about it.
If dry mouth is a factor, staying hydrated helps but doesn’t fully replace what saliva does. Sugar-free gum or lozenges containing xylitol can stimulate whatever salivary gland function remains. Saliva substitutes and mouth rinses designed for dry mouth can supplement natural protection, especially overnight when saliva flow naturally drops to its lowest levels.
Reducing the frequency of sugar exposure has a bigger impact than reducing the total amount. Consolidating snacks, avoiding prolonged sipping of sweetened beverages, and rinsing with water after eating are simple changes that shorten the window of acid attack on your teeth. For adults with gum recession, paying particular attention to brushing along the gum line with a soft-bristled brush protects vulnerable root surfaces without accelerating further recession.