Enamel, the outer layer of the crown of a tooth, is made of closely packed mineral crystals. Every day, minerals are lost and gained from inside the enamel crystals. Losing minerals is called demineralization. Gaining them back is called remineralization.

Demineralization begins with the bacteria in the plaque on your teeth. They feed on sugar and other carbohydrates in your mouth and produce acids. The acids dissolve crystals in tooth enamel. The loss of enamel is balanced by remineralization. In this process, minerals in the saliva, such as fluoride, calcium and phosphate, are deposited back into the enamel. Too much loss of minerals without enough replacement leads to tooth decay.

Fluoride helps teeth in two ways. When children eat or drink fluoride in small doses, it enters the bloodstream and becomes part of their developing permanent teeth. Swallowed fluorides also become part of the saliva and strengthen teeth from the outside. Acids are less able to damage tooth enamel strengthened by fluoride.

Can Fluoride Cause Harm?

Fluoride is safe and effective when used properly.

All children should use fluoridated toothpaste. If your children are younger than 6, be cautious about how they use it, however. Young children are more likely to swallow toothpaste after brushing instead of spitting it out. Use only a pea-sized amount of toothpaste when they brush. Encourage them to spit out as much as possible. Avoid flavored toothpastes that might encourage swallowing.

Swallowing toothpaste while teeth are developing can cause a cosmetic defect known as fluorosis. Mild fluorosis appears as white specks on the tooth. For many people this is not noticeable. Swallowing larger amounts of fluoride can cause “mottled” brown enamel. This is unusual and occurs mainly in areas that have naturally high levels of fluoride in the water.

How to use Fluoride

You can get the benefits of fluoride from different places. It can work from an external source and from the inside of your body. To work the best, you need to get it from both. At home, you and your family should brush with fluoride toothpaste at least twice a day for two minutes, especially after eating breakfast and before bedtime.

Reference: http://www.colgate.com/en/us/oc/oral-health/basics/fluoride/article/fluoride-and-your-teeth