Optimally administered Fluoride reduces, but does not completely eliminate dental decay. Dental decay is caused by the action of resident dental bacteria with carbohydrates (sugars, and starches) to produce acid directly against the surface of the teeth. Fluoride works by reducing the chemical suseptibility of enamal to acid attack. While the bacteria can work with about any carbohydrate, acidic drinks (soda, lemon tea, etc), sugary foods (soda again, hard candy, sticky candy, sugary snacks, etc) greatly accelerate the process.
Proper and frequent toothbrushing and particularly flossing are very helpful to decrease bacterial populations. Children often crave sweet sugary foods or drinks, often have poor oral hygiene habits, and I usually suggest topically applied fluoride to help protect their teeth. But they are not alone! "Dry mouth" is common with senior Americans especially women. "Dry mouth" can be caused by medications, radiation treatments, autoimmune diseases, or simply age, and it can result is very high decay rates. I see this every week in my practice. Strokes, dexterity issues, and vision problems can affect oral hygiene and also contribute to dental problems in our elders. Patients with exposed root surfaces are more suseptible to decay and may benefit from topical fluoride application. Anyone with "new" area of decay (as opposed to replacing a worn out filling) should consider topical fluoride.
Water fluoridation does not eliminated the need for dental treatment of for topical fluoride, but it does greatly reduce its incidence and the cost of dental services.
Thyroid problems due to properly administered water fluoridation. Thyroid disease is a serious issue. There was a Mexican report that excessive fluoride intake via fluoridated salt could affect Thyroid function. There are no scientifically valid studies that show thyroid problems due to properly administered water fluoridation. Fluoridated salt was used where central water treatment was not available in an effort to control rampant tooth decay. It is harder to control the individuals fluoride intake this way and there is no adjustment for local background fluoride levels. Perioral acne due to fluoride.
There are reports of acne around the mouth that patients relate to fluoridated water and/or fluoridated toothpaste. Typically physcians will say that allergic responses are usually to protein compounds rather than ions like fluoride. Sodium laural sulfate (surfactant), or the flavoring agents such as cinnamon or the various mints are credited with occasionally causing issues with sensitative patients. I usually suggest switching brands of tooth paste in my patients if problem occur.