Cavities: How They Form and How to Prevent Them

Written by Dr. Nick Jadidi, DDS — Last updated November 13, 2026 · 7-minute read

This article is general information for our patients, not a diagnosis. If something in your mouth hurts, changes, or worries you, have it examined.

A cavity isn’t bad luck. Decay moves through stages you can learn to spot, and much of what happens next comes down to daily habits.

A white spot, a twinge or a brown groove is only a clue. Whether decay needs treating comes clear when we read your history, the tooth, specific tests and an X-ray together.

What’s actually happening when a tooth decays

A cavity is damage made by acid from plaque bacteria, dissolving the tooth’s hard surface faster than saliva can rebuild it. It isn’t the sugar touching the tooth that does the harm, and it isn’t “soft teeth” you were born with.

Plaque is the soft, sticky film that builds back up between cleanings, and it is mostly bacteria. Two kinds do most of the damage (Streptococcus mutans and lactobacilli). When you eat anything sugary or starchy (fermentable carbohydrate), they feed on it and give off acid. That includes the sugar in juice and candy, and the starch in crackers, chips, bread and cookies once saliva breaks it down.

Enamel, a tooth’s outer shell, is a crystal of calcium and phosphate. When acid sits on it, those minerals leak out (demineralization). Saliva and fluoride push minerals back in every day (remineralization). A cavity forms only when loss outpaces repair for long enough.

Why how often you eat sugar matters more than how much

The first thing I weigh in someone getting new cavities is how often sugar reaches the teeth, not how much they eat. Each sugary hit lets plaque acid attack the enamel for 20 to 60 minutes, so a chocolate bar finished in five minutes does less harm than the same sugar sipped all afternoon.

Enamel starts to dissolve once plaque acid crosses its tipping point, around pH 5.5 (pH is the acidity scale; lower is more acidic). You reach it within minutes of eating. Saliva then washes the sugar away, neutralizes the acid and lays minerals back down, which takes 20 to 60 minutes. Sip a sweet coffee over two hours or graze all day, and each sip restarts the attack before recovery finishes. Eat the same sugar at meals and you get a few short attacks with long gaps between.

So the most useful change isn’t cutting out every treat. Keep sugary and acidic things with meals, drink water rather than juice or pop between them, and keep between-meal sugar to about twice a day at most. Past that, the risk of new decay climbs.

The white spot: where decay can still go either way

One early sign I look for is a dull, chalky patch on the enamel (a white spot lesion), where minerals have been lost just under an intact surface. Because the surface hasn’t broken, decay at this stage can often be stopped, or even reversed, with fluoride and better cleaning before any drill.

What I’m reading in that spot is whether it’s still losing ground or has stalled:

I wouldn’t judge that from a glance alone. The point of no return is when the weakened surface collapses into a real hole (cavitation) that reaches the softer layer under the enamel (the dentine). That layer doesn’t grow back, so the tooth then needs a filling. A fresh filling that feels tender at first is usually normal; see our note on sensitivity after a new filling.

How fluoride actually helps

Fluoride does most of its work on contact with the tooth, not by being swallowed. It slows mineral loss during an acid attack, speeds the repair afterward, and helps the tooth rebuild with a tougher crystal (fluorapatite) that only gives way to much stronger acid, around pH 4.5 instead of 5.5. A fluoride-strengthened surface resists milder attacks that would harm bare enamel, and it makes life harder for the acid-producing bacteria too.

That’s why I say spit, don’t rinse, after brushing. Rinsing washes off the thin film of fluoride that would otherwise keep working a while longer.

Fluoride toothpaste for a family: how much, and is it safe

Regular drugstore fluoride toothpaste in Canada runs about 1,000 to 1,100 ppm (the strength in the fine print), and one family tube suits every age; only the amount changes. From age three, a pea-sized blob twice a day is plenty for children and adults. For under-threes, check with us first: the Canadian Dental Association advises fluoride toothpaste only when a toddler’s decay risk calls for it, and for a low-risk toddler a rice-grain smear or a wet brush does the job. Supervise young children until around age seven so they use the right amount and spit rather than swallow. In pooled trials, brushing twice a day rather than once meant about 14% less decay, and an adult supervising prevented roughly a further 11%. (Stronger pastes, up to 5,000 ppm, are prescription-only for high-risk patients.)

Fluoride has decades of research behind it. Its one real downside is faint white flecks on the enamel (dental fluorosis), which can appear if a young child takes in too much while the adult teeth are forming, mostly in the first three to four years. At family amounts it is almost always mild and cosmetic. It comes from regularly swallowing toothpaste or stacking several fluoride sources, not from a pea-sized amount or normal tap water. For almost everyone the benefit clearly wins. For the longer answer on whether fluoride is safe, see our separate guide.

For teeth that need more help, we can paint on a professional fluoride varnish in the chair. It is far stronger than toothpaste, around 22,600 ppm, and sets hard on the surface. We usually apply it two to four times a year for higher-risk patients. Afterward you can eat soft food after about 30 minutes, but wait at least four hours before brushing or chewing anything hard.

Sealants, and who they help most

The chewing surfaces of back teeth have narrow pits and grooves (fissures), some thinner than a toothbrush bristle, so brushing can’t reach the bottom and most childhood cavities begin there. A sealant is a thin coating flowed into those grooves that hardens into a smooth surface, with no drilling and no freezing.

Sealants help most on children’s permanent molars and on the back teeth of anyone with deep grooves or a tendency to decay. They guard only the grooved top, not the sides between teeth, so brushing, flossing and sensible snacking still matter, and we check them each visit. We usually place them as part of routine fillings and cleanings for kids, and they often come up at your child’s first dental visit. For eligible kids they’re among the preventive services covered under the Canadian Dental Care Plan. For materials, timing and how long they last, see how dental sealants work.

How we find a cavity — and when you truly need a filling

Much decay hides where the eye can’t reach, especially between teeth that touch, which is what bitewing X-rays are for. They show early decay before anything is visible or sore. Adding bitewings to a visual exam picks up two to eight times as many cavities between the teeth.

How often you need X-rays depends on your cavity risk: your history of cavities (the strongest predictor of future ones), how often you have sugar, how dry your mouth runs, and how well plaque is kept off. Low risk might mean bitewings every year or two. Someone actively getting cavities may need them every six months, with fluoride and checkups closer together.

No single clue tells me whether to watch or to fill. While decay is still in the enamel or sitting as a white spot, I’d rather monitor and tip the odds toward repair with fluoride, cleaning and maybe a sealant. Once it has become a hole reaching the softer layer, that structure is gone for good. A filling then cleans out the decay and rebuilds the tooth before the hole reaches the nerve. Keeping plaque off, including a professional cleaning and scaling on the schedule we set, keeps most teeth on the monitoring side of that line.

Does the plan cover fillings?

Yes — fillings are among the everyday treatments the Canadian Dental Care Plan is built to help with, though what applies to you depends on your eligibility and your plan year. The plan works out its share against a set amount for each treatment rather than paying a flat percentage of whatever a practice charges, so a portion can still be yours; the honest answer for your own mouth comes from checking your own coverage rather than from a general rule. We accept the plan here in Barrie and bill it directly, so you are not left chasing the paperwork. Call us at 705-721-9229 with your details before treatment and our team will check what applies and put it in writing first — or book online. For who qualifies and how the plan works overall, see the Canadian Dental Care Plan explained.

Not sure where your own teeth, or your child’s, stand? Reach Prince William Way Dental in Barrie through our contact page and we’ll find a time. This article is part of our Adult Patient Information library.