Tap water

Frequently Asked Question: Does Fluoride Cause Cancer?

In our "Research FAQs" series of articles, the Irish Cancer Society's Head of Research Dr Robert O'Connor explores some of the most frequently asked questions that our Research Team get asked by the general public.

This is another of our most common queries and with many myths circulating, one that is actually quite easy to provide reassurance on. In short, the fluoride in our tap water (and toothpaste) is not a cause for concern when it comes to cancer.

Some background

What is fluoride?

Fluorine is a common element in many things around us. Chemically it belongs to a group of elements which include Chlorine and Iodine. Fluoride is the salt form of the element fluorine. Each of these elements share the characteristic of being highly reactive and liking to bond with other elements, like carbon. In free or acid form, that reactivity makes them very dangerous but when they do bind they make very strong bonds which become extremely unreactive. By way of example, the related element Chlorine is the reactive part of common household bleach and Hydrochloric acid (both very dangerous) but our bodies are also made up of large amounts of Chlorine (Chloride) and we consume it in substances like salt (sodium chloride). So Chlorine is vital to health. Fluorine is a component of lots of substances around is. For example, roughly 1 in 5 of all medicines have Fluorine in them, because Fluoride in such substances can give very useful properties.

Why is fluoride added to water?

Fluoride is very commonly found naturally dissolved in water across the globe. When water filters down through various rocks it tends to dissolve fluoride salts. Hence, water from certain types of wells, aquifers etc (ground water) will sometimes contain proportionately high amounts of dissolved fluoride. Much of the public water supply in Ireland comes from surface run off (rivers, lakes etc) and hence tends to have much lower fluoride levels unless supplemented.

Fluoride was identified as a component of human bones and teeth in the 19th century. In the early part of the 20th century a number of dentists and dental researchers examined tooth decay rates. They found associations between low levels of fluoride and high rates of decay. Conversely, in a region of Colorado, it was common to see staining on teeth. This was found to reflect higher than normal concentrations of fluoride in the teeth (from local drinking water) and these teeth (and especially the stained areas) were very resistant to decay.  There were no other untoward health effects seen when fluoride was found at high levels in public water.  Some fairly crude experiments with supplementing fluoride into public water schemes showed clear and measurable decreases in levels of tooth decay. Hence, many countries with low fluoride levels in their water adopted water fluoridation employing a fluoride-containing salt that is mined from certain rocks. Fluoride has also been supplemented in other materials, eg milk or table salt, but most commonly we see it in toothpaste. Fluoride becomes concentrated in saliva and coats in tiny amounts on to the surface of teeth making them less reactive to acidic by-products of bacteria which can grow in our mouth. Since everyone drinks water, addition of fluoride is a simple, effective and equitable public health measure to reduce tooth decay (and associated diseases) irrespective of financial means. The increased use of toothpaste and regular brushing also accomplishes the same outcome but this tend to benefit more those who are wealthier.

What does the evidence say?

There have been quite a few studies of cancer, examining associations between fluoride and risks of certain cancers. Some references to articles from professional organisations such as the Health Research Board (HRB), Centres for Disease Control (CDC) are listed below. No evidence has been found to support fluoride having any effect on general cancer levels or deaths. As fluoride can be found in bone as well as teeth, cancer of the bone has been a particular research focus.  One laboratory study suggested a possible link. Human studies have suggested extremely small differences both for and against a role for fluoride in bone cancer. This is very common in studies looking at something where there is no difference. In other words, some studies find a tiny positive effect, some find a small negative effect. This is described as “regression toward the mean” and suggests that fluoride has no role in cancer. If there is a role, it is extremely small effect and likely to favour a lower rate of bone cancers in those drinking fluoridated water.


Taking all of this together, Fluoride is added to drinking water as part of measures to reduce tooth decay. There is no evidence that Fluoride in water or toothpaste has any effect on cancer risk.

Further Reading: