DEAR DOCTOR K:

I'm confused. Is fluoride harmful or not?

DEAR READER:

I know there are people who think fluoride in drinking water, toothpaste or mouthwash is harmful, so I'm likely to get some mail about this column. But my job is to tell you what the scientific evidence shows.

Fluoride is a powerful ally in your fight against tooth decay. As far as I have been able to determine, the rumors that abound linking fluoride to a broad range of ills -- from heart disease to allergies to genetic abnormalities -- have no scientific backing.

As with many things, fluoride can be lethal if you ingest excessive amounts. However, an adult would have to consume 5,000 to 10,000 glasses of fluoridated water in one sitting to reach dangerous levels. Certainly, young children could develop toxicity from fluoride if they swallow large numbers of improperly stored fluoride tablets, or ingest large amounts of a fluoridated toothpaste or mouth rinse. But there are lots of things around the home that can make children sick if they are not placed out of reach.

A minor drawback to using fluoride is the risk of fluorosis, a condition that discolors tooth enamel. Fluorosis appears in permanent teeth when a child ingests too much fluoride while these teeth are forming in the gums. The risk of fluorosis disappears once the permanent teeth are fully developed.

It also is true that some people are allergic to fluoride, but this is unusual. As with any allergy, the allergic effects go away when you stop taking the thing you're allergic to. Those people with allergies to fluoride need to be more careful about brushing and flossing, since their teeth are more vulnerable to cavities and gum disease if they can't take fluoride.

People of all ages can reduce their risk for tooth decay by regularly exposing their teeth to fluoride. Fluoride in the saliva enhances the body's ability to rebuild tooth enamel when acid-producing bacteria cause it to decay. This new enamel is actually harder and more decay-resistant than the original tooth surface. Fluoride makes it harder for plaque to stick to your teeth. It also makes it more difficult for bacteria to turn sugar into acid.

One of the simplest ways to get fluoride is from drinking water. You can also use toothpaste that contains fluoride. For most people, these two sources of fluoride are sufficient to keep decay in check.

Some communities don't have fluoridated water. Bottled water contains little or no fluoride, and fluoride is also removed from the water in some home water-treatment systems. In these cases, your dentist may suggest getting fluoride from other sources. You can get additional fluoride via mouth rinses, oral supplements, or treatments such as fluoride gels and varnishes applied by your dentist.

The bottom line: Don't be concerned about fluoride. In fact, the widespread fluoridation of drinking water is often cited as one of the 10 great public health accomplishments of the 20th century.

(Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.)

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