There’s a controversy going around whether the benefits of water fluoridation outweigh the potential harms.
What is the evidence for the harm this may cause and what evidence supports the health benefits?
Typically No, but it depends on the dose.
The World Health Organisation released a fairly comprehensive report (PDF Warning) called "Fluoride in Drinking-water" that covers this topic.
I apologise for quoting so extensively from this report, but, well, it's excellent. I'd suggest anyone that is skeptical, have a look at the report and the (many) papers it cites to justify its findings.
Firstly, fluoride is naturally occuring in all types of water.
The major risk cited by fluoride-in-water-skeptics is the risk of fluorosis. Unfortunately for them, this only occurs after excessive ingestion of fluoride. This typically occurs in regions that do not control the levels of fluoride in their drinking water and the levels are naturally high. Typically the effects are worse in hotter regions where people require more water.
What most of the governments in fluoridated-water countries do is attempt to find a middle ground between too much fluoride (causing fluorosis):
and too little (contributing to high levels of dental decay):
For a government, based purely on cost I would expect it's worth controlling the fluoride levels in drinking water. The cost of treating the water is lower than treating severe fluorosis or unnecessary dental caries, particularly if millions of people are likely to be affected.
The WHO therefore recommends that a moderate level of fluoride be maintained in drinking water:
The same report actually covers the (pretty ridiculous) claims of birth defects, cancer, etc. and debunks them. As they're not specifically claimed in the question, I'll leave it to people to read the report if they're interested.
I would suggest people also read @Oliver_C's post on a similar question for more information.
Skeptic would undoubtedly want to know about the systematic reviews done on fluoridation. They are listed here on the CDC website:
In short, these reviews find that fluoridation is cost-effective and safe. There are some recommendations for further research.
Item 1 Check out the 2006 National Research Council publication, Fluoride in Drinking Water: A Scientific Review of EPA Standards. Here is the brief: http://dels.nas.edu/resources/static-assets/materials-based-on-reports/reports-in-brief/fluoride_brief_final.pdf
Item 2 The vast majority of scientists were anti-fluoride in the beginning. Over the years, many leading pro-fluoridationists changed their opinions based on science.
Dr. John Colquhoun, Chief Dental Officer of Auckland was one. In this 1998 interview, he gives first hand account of how the data was manipulated to fit preconceived notions. Especially interesting is how the Hastings study changed methods and then threw out the control group because it didn't support the result they wanted: http://fluoridealert.org/fan-tv/colquhoun/
Item 3 People need to pay attention to KIDNEYS as well as cavities. Renal Failure and Fluoride in the Oxford Journal of Nephrology Dialysis Transplantation: http://ndt.oxfordjournals.org/content/23/1/411.1.full