According to the National Toxicology Program, "the preponderance of evidence" from laboratory 'in vitro' studies indicates that fluoride is a mutagen (a compound that can cause genetic damage).
It is generally accepted that if a substance can induce genetic damage there is a heightened risk that it could cause cancer as well.
While the concentrations of fluoride causing mutagenic damage in the in vitro studies is higher than the concentrations found in human blood, there are certain "microenvironments" in the body (e.g. the bones) where the concentrations of fluoride can accumulate to levels comparable to, or in excess of, those causing mutagenic effects in the laboratory.
Of particular concern are a series of studies indicating that fluoride can cause osteosarcoma (bone cancer) in both fluoride-treated male rats and boys under the age of 20 living in fluoridated areas. Osteosarcoma is a rare, but deadly, form of cancer that strikes primarily during the teenage years.
Of additional concern are recent studies indicating that: