This article is general information for our patients, not a diagnosis. If something in your mouth hurts, changes, or worries you, have it examined.
Most people treat a cavity as bad luck. It isn’t — decay is a process with clear steps, and once you can see them, a surprising amount of it is in your hands. Here’s how cavities form, what I watch for, and how we stop one before it needs a filling.
One thing first, because it matters. Nothing you can see or feel in your own mouth is the whole story. A white spot, a twinge, a brown groove — each is a clue that points somewhere, but what the decay is really doing, and whether it needs treating yet, only comes clear when we read everything at once: your history and cavity risk, a look at the tooth, how it responds to specific tests, and an X-ray that shows what the surface hides. Any one alone can mislead; putting them together is what settles it, and that part happens in the chair.
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’s alive, 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’s the sugar in juice and candy, but also the starch in crackers, chips, bread and cookies once saliva breaks it down.
The acid is the real problem. Enamel, a tooth’s outer shell, is a crystal of calcium and phosphate; when acid sits on it those minerals leak out (demineralization). But it runs both ways — saliva and fluoride push minerals back in (remineralization), every single day. 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 isn’t how much sugar they eat — it’s how often it reaches the teeth. 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.
The mechanism: enamel starts to dissolve once plaque acid crosses its tipping point, around pH 5.5 (pH is the acidity scale — lower is more acidic), which you reach within minutes of eating. Then saliva washes the sugar away, neutralizes the acid, and lays minerals back down — a recovery of 20 to 60 minutes. Sip a sweet coffee over two hours or graze all day, and each sip restarts the attack before that recovery finishes, so the tooth never leaves the danger zone. Eat the same sugar at meals and you get only a few short attacks with long gaps between.
So the most useful change most patients can make isn’t cutting out every treat. It’s clustering sugary and acidic things with meals, drinking water rather than juice or pop between them, and keeping between-meal sugar to no more than about twice a day — past that, the risk of new decay climbs.
The white spot: where decay can still go either way
One sign I look for early is a dull, chalky patch on the enamel (a white spot lesion). Minerals have been lost just under the surface while the surface itself is still intact — and because it hasn’t broken, this is often the stage where decay can be stopped, or even reversed, with fluoride and better cleaning before any drill. A white spot is a clue, not a sentence.
What I’m reading in that spot is whether it’s still losing ground or has stalled:
| What I check | Active (still losing minerals) | Arrested (decay has stopped) |
|---|---|---|
| How it looks | Dull and chalky | Shiny, sometimes brown or black |
| How it feels | Matte and rough | Hard and smooth |
| What we do | Tip the balance back with fluoride and cleaning | Usually leave it, even if it marks the tooth |
I wouldn’t settle that from a glance alone, though — how it looks, where it sits, your risk, and what the X-ray shows all factor in. The point of no return is when the weakened surface collapses into a real hole (cavitation) that reaches the softer layer beneath the enamel (the dentine). That layer doesn’t grow back, so once decay breaks into it the tooth needs a filling to rebuild what’s lost. 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, not by being swallowed. While it’s on the tooth 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. So a fluoride-strengthened surface shrugs off milder attacks that would have harmed 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 straight away 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 — what changes is the amount, not the paste. From age three, a pea-sized blob twice a day is plenty for children and adults alike; no need to cover the brush like in the ads. 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 one 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. Both habits earn their keep: 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, exist for high-risk patients, but those are prescription-only.)
The safety question comes up often. 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 still forming, mostly in the first three to four years. At family amounts it’s almost always mild and cosmetic, and it comes from regularly swallowing toothpaste or stacking several fluoride sources at once — not from a correct pea-sized amount or from normal tap water. Against how reliably fluoride cuts cavities, the benefit clearly wins for almost everyone.
For teeth that need more help, we can paint on a professional fluoride varnish in the chair — far stronger than toothpaste, around 22,600 ppm, set hard on the surface. We usually apply it two to four times a year for higher-risk patients; kids handle it easily, and afterward you can eat soft food after about 30 minutes but should wait at least four hours before brushing or chewing anything hard.
Sealants, and who they help most
The chewing surfaces of back teeth aren’t smooth — they’re cut with narrow pits and grooves (fissures), some thinner than a single toothbrush bristle, so no amount of careful brushing reaches the bottom. That’s where most childhood cavities begin: about 8 or 9 in every 10 cavities in school-age children start in these chewing-surface grooves. A sealant is a thin coating flowed into them that hardens and turns a plaque trap into a smooth surface — no drilling, no freezing, a few minutes per tooth.
Sealants help most on permanent molars, placed soon after they come in — around age six for the first set, twelve for the second — and on the back teeth of anyone with deep grooves or a tendency to decay. The usual material is a tooth-coloured resin; where a tooth is hard to keep dry, such as one only partly through the gum, we may use a glass ionomer that copes better with moisture. Two limits: a sealant guards the grooved top, not the sides between teeth, so brushing, flossing and sensible snacking still matter; and it can chip or wear, which is why we check it each visit and reapply if needed. With that upkeep, sealed permanent molars develop roughly 78% less decay on the chewing surface over the first two years, and about 60% less out to four years, than unsealed ones. We usually place them as part of routine fillings and cleanings for kids, and it often comes up at your child’s first dental visit. For eligible kids, sealants and fluoride are among the preventive services covered under the Canadian Dental Care Plan, so cost needn’t be the barrier it once was.
How we find a cavity — and when you truly need a filling
A lot of decay hides where the eye can’t reach, especially between teeth that touch. That’s what bitewing X-rays are for: small images showing those contact points and the early shadows of 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 — the whole reason we take them at sensible intervals, and how we judge whether one is deep enough to treat.
How often you need X-rays and checkups isn’t one-size-fits-all, so I run a quick read of your cavity risk — your history of cavities (the single strongest predictor of future ones), how much and 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.
And this is where the whole picture beats any single clue. A white spot, a sensitive tooth, a shadow on an X-ray — none alone 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 load the odds toward repair with fluoride, cleaning and maybe a sealant. Once it has collapsed into a hole reaching the softer layer, that structure is gone for good and a filling is the answer — to clean out the decay and rebuild the tooth before the hole reaches the nerve. Keeping plaque off, including a proper professional cleaning and scaling on the schedule we set, is what keeps most teeth on the monitoring side of that line.
Not sure where your own teeth, or your child’s, stand? We’re glad to take a look and walk you through it — reach Prince William Way Dental in Barrie through our contact page and we’ll find a time that works. This article is part of our Adult Patient Information library.
