Why the question has four different answers
Picture the decay front as water finding its way through different ground.
Through enamel it is slow. Demineralisation and remineralisation happen daily in both directions, and whether a lesion advances at all depends on which way that balance tips. In an adult with good saliva flow and regular fluoride, an enamel lesion can sit almost unchanged for years — exactly the lesions dentists monitor from check-up to check-up rather than drilling.
At the junction it accelerates and turns sideways. Where enamel meets dentine, decay spreads laterally along the boundary, which is why a cavity is routinely wider underneath than the small dark opening suggests.
Through dentine it is faster. Dentine is softer, less mineralised and shot through with tubules leading towards the nerve. Once bacteria are established here, progress is generally measured in months.
Into the pulp it is a different disease. When the nerve is involved you are no longer discussing a filling. The timescale becomes days, and the decision is between root canal treatment and losing the tooth.
German dental associations describe caries as a chronic disease with a highly individual course — a precise way of saying what every dentist sees: two people with the same X-ray finding can be in completely different situations a year later.
What makes your personal clock run fast
How often you eat is the biggest accelerator and the most modifiable. Each eating or drinking occasion triggers an acid episode, so the damage depends on how often the clock is reset rather than on the total sugar. Sipping sweetened coffee keeps the tooth surface acidic for most of the working day.
How much saliva you produce. Saliva buffers acid and returns minerals to the enamel. Dry mouth — from medication, mouth breathing, dehydration, stress or radiotherapy — removes the repair mechanism and can turn a slow lesion into a fast one.
Where the lesion sits. Between the teeth, in a deep fissure or under the margin of an old filling, plaque is undisturbed by any brush. If the gum has receded, decay attacks root dentine directly with no enamel to slow it — which is why root caries progresses faster.
Age of the tooth and its owner. Milk teeth have thinner enamel and a proportionally larger pulp, so decay reaches the nerve far sooner — a cavity that would be minor in an adult can become a painful abscess in a child within months. Newly erupted permanent molars are also more vulnerable, and fixed braces (feste Zahnspange) add dozens of plaque traps. Smoking and poorly controlled diabetes change saliva and the balance of oral bacteria.
What slows it down — including the window where it reverses
The good news is genuinely good, and it applies only at the beginning.
A white, chalky patch on an otherwise intact surface is decay that has taken minerals out of the enamel without breaking through it, and that process runs in both directions. Fluoride, thorough cleaning and fewer acid episodes per day can arrest the lesion and partly rebuild it. Nothing needs drilling. This is the entire argument for check-ups in a mouth where nothing hurts — the reversible stage is also the invisible one.
The window closes when the surface breaks. Once bacteria are in the dentine, the tooth has no mechanism to rebuild itself and the decay must be removed.
What keeps the clock slow: cleaning between the teeth once a day; fluoride toothpaste twice daily, spitting rather than rinsing; fewer eating occasions; taking dry mouth seriously; and regular examinations, so a lesion is watched rather than discovered late. Why decay starts in the first place is the subject of Why do I still get cavities even though I brush twice a day?.
"Can it wait?" — the situations people actually ask about
This is the practical heart of the question, and the answer depends on what you can observe.
Your check-up found an early lesion and the dentist wants to watch it. Normal practice, not negligence. Enamel lesions are monitored at intervals, with fluoride applied and cleaning improved in between. Keep the follow-up appointment — monitoring only works if the second look happens.
You have a visible hole but no pain. Weeks, not months. Nothing dramatic will happen next Tuesday, but the decay is in the dentine and will not stop on its own. Book within the next few weeks and the treatment is very likely still a filling.
A tooth reacts to cold and the feeling lingers. Days to a couple of weeks. The nerve is irritated, and the difference between treating this month and in three is often the difference between a filling and a root canal. The same applies if a piece of tooth or filling has broken off — see A filling has fallen out: what to do now.
You are flying home for three weeks. An early lesion will wait comfortably, and a visible cavity will not change dramatically in that time either. The risk is the timing of the pain: developing toothache away from your own dentist is an expensive way to save an appointment. If the tooth already reacts to cold or sweet things, have it seen before you fly.
Spontaneous pain, night pain, pain on biting, or swelling. No waiting. Same-day territory, and the out-of-hours service if it is a weekend.
You are new here and have not found a dentist yet. Register for a check-up before anything hurts; availability is far better for a routine examination than for an emergency. If language is the obstacle, English-Speaking Dentist in NRW sets out how to book and how the insurance side works for English speakers in Wuppertal and neighbouring towns such as Solingen and Remscheid.
Does decay spread from tooth to tooth?
Not in the way people usually imagine, but the underlying worry is not unfounded. The bacteria that cause decay are transmissible — classically from parents to small children by shared spoons or a dummy cleaned in an adult’s mouth, which is why prevention for children starts with the parents’ own mouths.
Between adult teeth the mechanism is simpler: two teeth touch, and the contact zone is the same environment on both sides. Where decay is found on one flank, the neighbouring flank is often affected too. Decay does not jump, but it does keep company.
What's normal — and when to book an appointment
Special situations
Children and milk teeth. Assume a faster clock: thin enamel, a large pulp and frequent snacking mean a small brown spot can become a painful tooth within months. Milk teeth also matter for the permanent ones behind them, so “it will fall out anyway” is not a treatment plan.
Pregnancy. More frequent eating, morning sickness and reactive gums all shift the balance. Check-ups during pregnancy are covered and recommended.
Dry mouth from medication. If you take daily medication and your mouth is dry, tell us. The prevention plan changes: higher-fluoride products, tighter check-up intervals, measures to stimulate saliva.
Several old fillings. Decay under an existing restoration is sheltered from your toothbrush and often progresses unnoticed — the topic of Decay under an old filling.
Only at the very first stage. While the damage is confined to the enamel and the surface is still intact — the chalky white spot — fluoride and better cleaning can arrest and partly reverse it. Once the surface has broken and bacteria are in the dentine, the tooth cannot repair itself.
In an adult, usually years from the first enamel damage, but only months once the decay is established in the dentine — and the range between individuals is very wide. In milk teeth it is considerably faster. Since nobody can measure your personal rate from the outside, the depth of the lesion rather than the calendar decides how urgent it is.
Yes, for early lesions that have not broken through the enamel. Monitoring with fluoride and improved cleaning avoids removing healthy tooth substance unnecessarily — every filling starts a cycle in which each replacement is larger than the last. The condition is that you attend the follow-up so the second look actually happens.
Measurably worse in three or four weeks, generally no — unless the tooth is already symptomatic. If it reacts to cold or sweet things, have it seen before you travel.
Not directly, but the bacteria are transmissible and the surfaces where two teeth touch share the same conditions. Decay on one side of a contact point often has a counterpart on the other, which is why both surfaces are examined and X-rays taken between the teeth.
Tooth decay is not a countdown but a balance that can tip either way, and its speed changes completely depending on the layer it has reached. In enamel it is slow enough to be monitored and, at the very start, reversible. In dentine it moves in months. In the pulp it becomes urgent. Since none of those stages announces itself reliably by pain, the only way to know which clock you are on is to have the tooth looked at — and the earlier you do, the smaller and cheaper the answer.
Zahnarztpraxis Tsypin · Wuppertal. This article was medically reviewed and is provided for information; it does not replace an examination. Updated August 2026.
