A report from the world’s oldest and most prestigious medical journal, The Lancet, has officially classified fluoride as a neurotoxin, in the same category as arsenic, lead, and mercury. 
The news was broken by author Stefan Smyle and disseminated by the Facebook page Occupy Food, which linked to the report published in The Lancet Neurology, Volume 13, Issue 3, in the March 2014 edition, by authors Dr. Phillippe Grandjean and Philip J. Landrigan, MD. The report can be viewed by clicking here.
Industrial chemicals identified
As noted in the summary of the report, a systematic review identified five different similar industrial chemicals as developmental neurotoxicants: lead, methylmercury, polychlorinated biphenyls, arsenic, and toluene.
The summary goes on to state that six additional developmental neurotoxicants have also now been identified: manganese, fluoride, chlorpyrifos, dichlorodiphenyltrichloroethane, tetrachloroethylene, and the polybrominated diphenyl ethers.
The authors added that even more of these neurotoxicants remain undiscovered.
ADHD, Dyslexia, and other cognitive impairments
In the Lancet report, the authors propose a global prevention strategy, saying that “untested chemicals should not be presumed to be safe to brain development, and chemicals in existing use and all new chemicals must, therefore, be tested for developmental neurotoxicity.”
Also in the report, they note that neurodevelopmental disabilities, including attention-deficit hyperactivity disorder, dyslexia, and other cognitive impairments, are now affecting millions of children worldwide in what they call a “pandemic of developmental neurotoxicity.”
The report coincides with 2013 findings by a Harvard University meta-analysis funded by the National Institutes of Health, which concluded that children in areas with highly fluoridated water have “significantly lower” IQ scores than those who live in areas with low amounts of fluoride in their water supplies. 
Fluoride also linked to cancers
Sodium fluoride in drinking water has also been linked to various cancers.
It is functionally different than the naturally-occurring calcium fluoride, and commonly added to drinking water supplies and used by dentists and in dental products who posit that it is useful for dental health.
Currently, fluoride is added to water supplies across much of North America, however, adding fluoride to drinking water is not always standard practice, being banned entirely throughout most of Continental Europe and in several other developed nations across the world.
 Woffinden, B. (2005, June 11). Fluoride water ’causes cancer’. Retrieved from https://www.theguardian.com/society/2005/jun/12/medicineandhealth.genderissues
 Mercola, J., Dr. (2013, January 28). Harvard Study Confirms Fluoride Reduces Children’s IQ. Retrieved from http://www.huffingtonpost.com/dr-mercola/fluoride_b_2479833.html