Fluoride and teeth

Fluoride is the most effective thing available for preventing tooth decay, and it does not work in the way it is usually described. The difference is not academic: it determines what is worth doing about it.

What actually happens at the tooth surface

Enamel is a mineral, and it is constantly dissolving and reforming. Bacteria in plaque feed on sugar and produce acid, and that acid pulls calcium and phosphate out of the enamel surface — demineralisation. Between meals, as the acid is neutralised, saliva puts mineral back — remineralisation. This happens many times a day in every mouth. Decay is not an event but the balance tipping one way for long enough, which is why how often sugar is eaten matters more than how much.

Fluoride tips that balance back, in three ways at once. It slows the rate at which mineral is lost. It speeds up remineralisation, and the mineral laid down when fluoride is present is a different, less soluble compound than the original enamel, so the repaired surface resists acid better than what was there before. And at higher concentrations it interferes with the plaque bacteria's ability to produce acid at all.

All of that happens at the surface, from the mouth side, while fluoride is in contact with the tooth. The older account — fluoride absorbed into the bloodstream, carried to the teeth and built into them as they develop — turns out to describe a minor part of the effect. What matters is topical contact, repeatedly, over a lifetime.

Two practical conclusions follow, and they are the whole of the advice:

• Brush twice a day with fluoride toothpaste, one of them last thing at night. Saliva flow drops during sleep, so the mouth loses its own defence for several hours; fluoride left on the teeth at bedtime is doing its most useful work.
• Spit out and do not rinse. Rinsing with water washes away the fluoride that was the point of brushing. Mouthwash immediately after brushing does the same thing, so it belongs at a different time of day.

It is also worth correcting a persistent confusion: the calcium and phosphate used to repair enamel come from saliva, which holds them in solution for exactly this purpose. They do not arrive from a glass of milk on its way past.

Where fluoride comes from in the UK

Toothpaste is the principal source, and the amount and strength depend on age. Current UK guidance sets out at least 1,000 parts per million for children under three, applied as a smear; more than 1,000 parts per million in a pea-sized amount for children of three to six; and 1,350 to 1,500 parts per million from the age of seven and for all adults. Children at higher risk of decay are advised 1,350 to 1,500 parts per million from the start. A single family toothpaste at around 1,450 parts per million, used in the right amount for the age, serves a whole household.

Toothpaste sold as fluoride-free, whatever else it contains, offers no protection against decay.

Fluoride varnish painted on by a dentist, hygienist or therapist is offered twice a year from the age of three, and more often for children at higher risk. Adults at higher risk can have it too.

A daily fluoride mouth rinse at 0.05% sodium fluoride is advised for children from about eight and for adults with active decay, used at a different time of day from brushing so that neither washes off the other.

High-strength prescription toothpaste at 2,800 or 5,000 parts per million exists for people with active decay, and is prescribed by age and circumstance rather than bought. It is a genuine option for anyone caught in a cycle of repeated fillings, and often not mentioned unless asked about.

Water supplies far less than most writing on the subject assumes. Only about a tenth of the population of England receives fluoridated water, with a little more from naturally occurring fluoride in some areas, and expansion schemes have been proposed in the north east. Scotland, Wales and Northern Ireland have essentially none. So in most of the UK, the tap is not a fluoride source, and advice built on the assumption that it is — common in older and in North American material — does not transfer.

Fluoride drops and tablets are not part of current UK advice for children, including in areas without fluoridated water. Toothpaste and varnish do the work.

Water fluoridation and what the evidence shows

The UK target where water is fluoridated is one milligram per litre, within the range the World Health Organization recommends. A 2025 evidence synthesis by the Academy of Medical Sciences reviewed what is known, and it is worth reporting honestly rather than as either a triumph or a scandal.

On benefit. In children, fluoridation at recommended levels is associated with roughly a quarter of a tooth fewer affected by decay, in both first and permanent teeth, compared with low-fluoride areas — and with hospital extractions for decay falling by about sixty per cent, which is the number that matters most, because those are young children having teeth removed under general anaesthetic. In adults the effect is small, around two per cent fewer decayed, missing or filled teeth, on weaker evidence.

The size of the benefit has fallen sharply over time. Before the mid-1970s, fluoridation was associated with around two more healthy teeth per person; in recent decades it is around a quarter of a tooth. The reason is not that fluoride stopped working but that fluoride toothpaste became universal in the meantime, so water is adding to a baseline that is already protected. That is the honest explanation of why the policy argument is less one-sided than it was.

On fluorosis. It becomes more common as exposure rises. At the levels used in the UK, fluorosis is generally mild and is not considered harmful.

On the other claims. No effect on IQ was found below 1.2 milligrams per litre. Some studies of concentrations above 1.5 milligrams per litre — above anything used here — have reported lower IQ, that evidence is contested, and further work is under way. At up to one milligram per litre, no link was found to Down's syndrome, kidney stones, hip fractures, or bladder or bone cancer.

The review also judged the overall quality of the evidence moderate at best, drawn mainly from studies of children. Anyone claiming the question is entirely settled in either direction is overstating what the research supports.

Fluorosis, and why the amount on the brush matters

Dental fluorosis is a change in the enamel caused by too much fluoride while the teeth are still forming, which means before about the age of six. It cannot develop in a tooth that has already come through.

At the mild end — which is almost all of it in the UK — it appears as faint white flecks or fine lines that most people never notice and a dentist has to look for. More pronounced forms, with brown mottling or pitted enamel, come from much higher exposures than anything in UK water or toothpaste.

In practice, the fluoride that causes fluorosis here is swallowed toothpaste, not water. That is the entire reason for the smear and pea-sized amounts, for supervising young children, and for keeping flavoured children's toothpaste out of reach of a toddler who regards it as a treat. Used in the recommended amounts, toothpaste prevents a great deal of decay and causes no visible fluorosis.

Swallowing too much at once

Acute fluoride toxicity is a different matter from fluorosis and comes from swallowing a large quantity in a short time — realistically, a small child eating a substantial part of a tube of toothpaste. It causes nausea, vomiting, abdominal pain, diarrhoea and thirst.

Toothpaste belongs out of reach of small children for this reason. Where a child has eaten a quantity of it, milk helps bind the fluoride, and NHS 111 or a pharmacist can advise on whether anything further is needed.

Who benefits most

Fluoride helps everyone, but some people are losing the demineralisation battle badly enough that the extra measures are worth asking about: anyone with a dry mouth, whether from medication, a medical condition or treatment; anyone with a history of repeated fillings; anyone wearing a fixed orthodontic appliance, which traps plaque; anyone with exposed root surfaces from gum recession, since root surface decays faster than enamel; and older people, in whom root decay is common and often missed.

See also cleaning teeth and gums, tooth decay and cavities and dental care for babies.