I have heard claims that it is unsafe for those with osteoporosis to drink tap water in municipalities that add fluoride to it, because it increases risks of fractures. For example:
short-term high-dose fluoride studies show the same amount of fluoride accumulates in the bones of osteoporosis patients as would be found in some people who are chronically exposed to long-term "low" doses of fluoride (such as in fluoridated areas). People with renal insufficiency, for example, can incorporate four times more fluoride into bone than an average healthy individual and would therefore be more susceptible to the long-term effects of drinking "optimally" fluoridated water than the average individual
Supporting this are human studies performed, given therapeutic doses of fluoride to try to prevent fractures from osteoporosis, which causes low bone density, often have found increases in fracture rates in the treated patients, even though their bone density increased.
So, the important scientific question is whether water fluoridation can lead to high enough levels of fluoride in your bones to noticeably weaken them. A dozen or so epidemiological studies have investigated this, with mixed results. Some of them show that fairly low levels of fluoride intake can increase the risk of fractures, whereas others have found no effect.
Trying to research, I came across an older journal article from 1996 stating that studies on the general population have been inconclusive.
Data on the relationship between fluoride intake and hip fracture risk at the individual level, and data relating fluoridation to bone mineral density are required. Until these become available, the burden of evidence suggesting that fluoridation might be a risk factor for hip fracture is weak and not sufficient to retard the progress of the water fluoridation programme.
Is fluoridated water actually unsafe for those with osteoporosis?