If you brush twice a day and still get told you need a filling, the explanation is usually one of five things, and none of them is that you are lazy about brushing. Decay is not a punishment for poor effort. It is the result of a chemical balance tipping the wrong way, and brushing only controls part of that balance. The most common reasons diligent brushers still get decay are that a toothbrush cannot reach between the teeth, that frequency of sugar and acid matters more than amount, that reduced saliva removes the mouth’s own repair mechanism, that certain surfaces are simply harder to keep clean, and that decay under and around older dental work is invisible from the outside.
This article explains what is actually happening at the tooth surface, then works through each cause and what can be changed about it.
How does tooth decay actually happen?
Decay is not a hole that appears. It is a process that runs in both directions.
Your teeth are covered in a thin film of bacteria called plaque. When you eat or drink anything containing fermentable carbohydrate, which includes sugar but also bread, crackers, chips and fruit, those bacteria produce acid as a by product. The acid lowers the pH at the tooth surface, and while the pH is low, mineral leaves the enamel. This is demineralisation.
Saliva then goes to work. It buffers the acid back towards neutral and delivers calcium and phosphate back to the tooth surface, which restores mineral to the enamel. This is remineralisation. Fluoride, whether from toothpaste or fluoridated water, makes that repair process more efficient and produces a mineral structure that is more resistant to the next acid attack.
A healthy mouth cycles between these two states all day and stays roughly in balance. Decay happens when the demineralising side of the ledger consistently outweighs the remineralising side, over months and years. Every cause below is a version of the same thing: more acid time, less repair time, or plaque left undisturbed somewhere specific.
Reason 1: a toothbrush cannot clean between your teeth
This is the single most common explanation, and it is a mechanical limitation rather than a technique problem. Bristles clean the surfaces they can physically contact. They do not reach the contact points where two teeth touch each other, and they do not reach far below the gum line.
That matters because the space between two adjacent teeth is a near ideal environment for decay. It is sheltered, food packs into it, saliva flow through it is limited, and the plaque there is rarely disturbed. A great many adult fillings are placed on surfaces that face another tooth, which is precisely the area a brush was never going to reach.
It is also why these cavities are found at a check-up rather than noticed at home. You cannot see the surface, and it does not usually hurt until it is well established.
What to change
- Clean between the teeth once a day, using floss, interdental brushes, or both depending on the size of your spaces.
- Interdental brushes are often easier to use consistently where there are slightly larger gaps, recession, bridges or implants. Floss suits tight contacts.
- Ask which is appropriate for your mouth at your next appointment rather than guessing, because the right tool varies from person to person and even between different areas of the same mouth.
- Consistency beats technique perfection here. Daily and imperfect outperforms occasional and thorough.
Reason 2: how often you eat matters more than how much
This is the point that changes outcomes most often, and it runs against how most people think about sugar.
Each separate exposure to fermentable carbohydrate produces an acid episode, and the recovery afterwards takes time. One serve of something sweet with a meal is one episode, and the mouth then has hours to recover. The same quantity spread across an afternoon of grazing produces a near continuous low pH, and the tooth surface never gets a repair window.
In practical terms, a person who eats dessert after dinner and drinks water the rest of the day is at lower decay risk than a person who eats less sugar in total but sips a sweetened drink from a desk bottle all day.
The patterns that quietly cause problems
- Sipping soft drink, cordial, sports drink, energy drink, fruit juice or sweetened coffee across a work day
- Grazing on crackers, chips, muesli bars or dried fruit between meals
- Lollies, mints or throat lozenges held in the mouth, which extend the exposure by design
- Anything sugary last thing at night, because saliva flow drops during sleep and the recovery mechanism is at its weakest
- Sugar free carbonated drinks, which do not feed the bacteria but are still acidic and contribute to erosion
What to change
- Reduce the number of separate eating and drinking occasions rather than only reducing quantity.
- Make plain water the default between meals. In fluoridated areas, tap water contributes as well.
- If you want something sweet, have it with a meal rather than alone between meals.
- Avoid anything other than water after brushing at night.
Reason 3: dry mouth removes your repair mechanism
Saliva is doing most of the defensive work in your mouth, and people rarely connect a dry mouth with their teeth. Reduced saliva flow raises decay risk substantially, because the buffering and remineralising side of the balance is weakened while the acid side continues unchanged.
Common contributors in adults include a number of prescribed medications, including some used for blood pressure, depression, anxiety, allergies, pain and bladder control, as well as certain medical conditions, dehydration, alcohol, smoking, and habitual mouth breathing including during sleep.
This is frequently the missing explanation when someone who has had no problems for decades suddenly starts getting decay in their fifties or sixties. Nothing about their brushing changed. Their medication list did.
What to change
- Mention dry mouth at your appointment, including waking with a dry mouth. It changes both the risk assessment and the recall interval.
- Do not stop or alter any prescribed medication on your own. The approach is to protect the teeth around the medication, not to remove it.
- Sip water regularly through the day rather than a flavoured drink, which solves the dryness without adding acid exposure.
- Ask about additional fluoride measures if dry mouth is ongoing.
Reason 4: technique and timing details that undo good brushing
A few habits are common and quietly counterproductive.
Rinsing after brushing
Rinsing your mouth with water straight after brushing washes away the fluoride you have just applied. Spitting the excess out and not rinsing leaves it on the tooth surface, where it continues to work. This is one of the simplest changes available and one of the most frequently missed.
Brushing immediately after acid
Enamel is temporarily softened after acidic food or drink, including citrus, soft drink, wine, sports drinks and after vomiting or reflux. Brushing while it is softened removes surface material that would otherwise have re-hardened. Rinsing with plain water and waiting before brushing is the better sequence.
Brushing too hard, or with too firm a brush
Force does not improve plaque removal, and over years it contributes to gum recession and wear at the gum line. A soft brush angled towards the gum line at roughly 45 degrees does the job. Exposed root surface caused by recession has no enamel on it and decays more readily than the crown of the tooth, so heavy handed brushing can create the conditions for the exact problem it was meant to prevent.
Missing the same areas every time
Most people clean the outer surfaces of their front teeth well and the inner surfaces of their back teeth poorly, and they do it the same way every day. Following a set order around the mouth each time is a small change that closes those blind spots.
Reason 5: some surfaces are simply harder to keep clean
Anatomy and dental history both affect risk, and neither is a matter of effort.
- Deep grooves and fissures on the biting surfaces of back teeth can be narrower than a toothbrush bristle, so plaque sits in them regardless of technique.
- Crowded or overlapping teeth create sheltered areas that are difficult to access with any tool. This is one of the practical arguments for addressing alignment beyond appearance.
- Exposed root surface following gum recession has no enamel and is more vulnerable than the crown of the tooth.
- Margins around older fillings and crowns are a common site for new decay. The restoration itself does not decay, but the natural tooth it joins does, and that junction is hard to clean and impossible to inspect at home.
- Wisdom teeth are often partly erupted and positioned where cleaning is genuinely difficult.
- Orthodontic appliances, including fixed retainers, create additional surfaces that trap plaque.
If any of these apply to you, the answer is usually a shorter recall interval and a targeted cleaning approach rather than more vigorous brushing.
Is erosion the same thing as decay?
No, and it is worth separating them because they get confused constantly.
Decay is bacterial. Plaque bacteria produce acid locally, in specific sheltered spots, and the damage is localised to those areas.
Erosion is chemical and does not need bacteria. Dietary acid, reflux or repeated vomiting dissolves enamel across broad surfaces. Eroded teeth tend to look thinner at the edges, more yellow as the underlying dentine shows through, and the biting surfaces can appear cupped or dished.
They are different processes but they compound each other. Erosion thins the enamel that would otherwise resist decay, and both respond to reducing the frequency of acid exposure.
What does early decay look like, and can it be reversed?
Early enamel decay often appears as a chalky white patch rather than a dark hole, and it is easiest to see near the gum line when the tooth is dried. At that stage the enamel has lost mineral but the surface has not collapsed, and it can often be arrested or remineralised with improved plaque control, reduced acid frequency and increased fluoride exposure.
Once the surface breaks down and decay reaches the dentine underneath, it will not reverse and a restoration is required. Dentine is softer than enamel and decay generally progresses faster once it gets there. If it reaches the nerve, the conversation changes again.
This is the practical reason regular examination matters even when nothing hurts. The stage at which decay is still reversible produces no symptoms at all.
So why did I get a filling when my friend never does?
Decay risk is not evenly distributed, and it is not a fair reflection of effort. Two people with identical brushing routines can have very different outcomes because of saliva flow and composition, snacking patterns, medications, fluoride exposure across their lifetime, tooth anatomy, crowding, existing dental work, and the bacterial makeup of their plaque.
The useful response is not more brushing. It is identifying which of the factors above actually apply to you, and adjusting those.
A practical checklist
- Clean between your teeth daily, with the tool that suits your spaces.
- Spit after brushing, do not rinse.
- Wait after acidic food or drink before brushing.
- Use a soft brush, angled at the gum line, following the same order each time.
- Reduce the number of separate sugar and acid exposures across the day.
- Nothing but water after brushing at night.
- Raise dry mouth, reflux, grinding and any medication changes at your appointment.
- Keep to the recall interval your dentist has set, because early decay is silent.
Talking it through in Ballarat
If you have been doing the right things and still getting bad news, an appointment focused on working out why is a reasonable thing to ask for. You can book online at Smile Creative or call the studio on 03 4320 0777. Dr Karishma Wijeyesinghe personally treats all patients at the Ballarat Central studio, and general and preventative treatments are outlined on the general dental treatments page.