Safety and effectiveness
Sixty years of reputable scientific studies clearly show water fluoridation is safe at the recommended optimal levels. Too much of anything that the body needs, like salt, sunlight or fluoride, can cause harm. There is no risk of getting too much fluoride from drinking fluoridated water.
Some countries, including parts of China, India, Africa and the Middle East, have excessively high levels of naturally occurring fluoride in their drinking water. These high levels (which can be well over 10mg/L) have been found to cause damage to bones (skeletal fluorosis) and teeth when consumed over many years. These problems do not exist in New Zealand.
Fluoride is added to water supplies in accordance to specific quality standards. The Maximum Acceptable Value (MAV) of fluoride in drinking water is set at 1.5mg/L. The amount of fluoride to be added is calculated daily and a weekly water sample is sent for external testing at a Ministry of Health approved laboratory.
Slight white flecking of teeth (known as mild dental fluorosis) has been associated with water fluoridation. This is only a cosmetic issue and would mostly only be noticeable to a dentist. There are also many other causes of flecking or spots on teeth. The 2009 New Zealand Oral Health Survey reported no significant differences in the levels of dental fluorosis of people living in fluoridated and non-fluoridated areas, and importantly, no cases at all of severe dental fluorosis in the people examined.
For more information about the safety and effectiveness of water fluoridation:
Fluoridation questions and answers - Ministry of Health
Water Fluoridation: Questions and Answers - Department of Health, Victoria
A Systematic Review of the Efficacy and Safety of Fluoridation - National Health and Medical Research Council, Australia