Brushing twice a day is a good habit, but it is only one part of what decides whether a tooth decays. Decay happens when plaque bacteria turn sugars into acid that draws minerals out of the enamel. A brush cleans most surfaces well, yet it cannot reach between teeth, and it does nothing about how often you eat sugar, how much saliva you make or how deep the grooves in your teeth are.
So new cavities despite regular brushing usually point to one of a few causes. Finding yours is more useful than brushing harder.
The surfaces a brush cannot reach
Where two teeth touch, the bristles cannot get in. Plaque sitting in that contact area is left alone day after day, and decay often starts there, hidden from view until it shows on an X-ray. Floss or interdental brushes are the tools that reach those surfaces. Research on cleaning between teeth has measured gum inflammation rather than decay: a Cochrane review (Worthington and colleagues, 2019) found no trials that assessed decay between teeth, and low to very low certainty evidence that adding floss or interdental brushes to brushing may reduce gum inflammation, with interdental brushes also possibly reducing plaque. Our article on cavities between teeth explains why this type of decay is so easy to miss.
How much sugar, and how often
Each time you have something sugary, plaque bacteria make acid for a while afterwards. A soft drink sipped over an hour, a packet of lollies eaten one at a time or several sweetened coffees through the day keep that acid coming back before the mouth has recovered. Spreading the same amount of sugar across the day tends to do more harm than having it with a meal.
The total amount matters too. A systematic review that updated the evidence behind the World Health Organization's sugar guideline (Moores and colleagues, Journal of Dental Research, 2022) found moderate-quality evidence that decay is lower when sugars make up less than 10% of daily energy intake. Keeping sweet foods and drinks to mealtimes and choosing water in between helps on both counts. Our guide to how decay forms and how to prevent it covers this in more detail.
Less saliva, less protection
Saliva rinses food away, neutralises acid and carries calcium and phosphate that help repair early enamel damage. When there is less of it, decay can move faster even if your brushing is good. Many common medicines can reduce saliva, including some antidepressants, antihistamines and blood pressure medicines. Breathing through your mouth, especially at night, and not drinking enough water can also leave the mouth dry.
If your mouth often feels dry, tell your dentist or OHT and bring a list of your medicines. Do not stop or change a medicine on your own; your doctor can review it if needed, and your dental team can suggest ways to protect your teeth in the meantime.
Teeth that are harder to keep clean
Some teeth are simply more difficult to clean. Back teeth with deep, narrow grooves hold plaque the bristles cannot reach, and fissure sealants can cover those grooves in suitable teeth. Crowded or overlapping teeth create extra hiding spots. Edges of old fillings and crowns can collect plaque, and gums that have receded expose root surfaces, which are softer than enamel and decay more easily.
These are not things you can fix by brushing longer. They are reasons to clean more carefully in particular spots and to have the area checked regularly.
Fluoride and brushing habits
Fluoride helps enamel resist acid and repair early damage. Brush twice a day with a fluoride toothpaste, and spit out the excess afterwards instead of rinsing with water, so some fluoride stays on the teeth. A dentist or OHT may suggest a higher-strength fluoride toothpaste or a fluoride varnish if you are at higher risk.
Scrubbing harder does not remove more decay-causing plaque between teeth, and it can wear gums and the softer root surface. A soft brush used gently and thoroughly works better. Replace your toothbrush about every three months, or sooner if the bristles splay; our guide on how often to replace a toothbrush has more on this.
Early decay can sometimes be stopped
Not every sign of decay needs a filling. When it is found early, before the surface has broken down into a hole, fluoride, better cleaning and less frequent sugar can sometimes stop it progressing. Our article on whether early decay can be reversed explains what that involves and how it is monitored.
Questions patients ask
Can I get cavities even if I brush well?
Yes. Brushing does not reach between the teeth, and sugar habits, dry mouth and tooth shape all affect your risk. Cleaning between teeth and having sugar less often are usually the most useful changes.
Why do my cavities keep appearing between teeth?
Because the brush cannot get into the contact points, plaque can stay there undisturbed. Floss or interdental brushes clean those surfaces, and X-rays help find decay there early.
Is drinking soft drink through the day worse than having it with lunch?
Usually, yes. Sipping keeps the teeth under repeated acid attack, while having it with a meal limits the number of times the acid returns.
Working out why it keeps happening
If you keep needing fillings despite your best efforts, a check-up can look at where the cavities are forming and what is likely behind them, from diet and dry mouth to hard-to-clean teeth. Our dentists and OHTs at Lumi Dental focus on prevention as well as treatment. See current new patient options on our offers page or read about our general dental care in Melrose Park.
This article is general information only and is not a substitute for personal dental advice. Please see a dentist to discuss your own situation.
