Medical experts have called for more research into the risks of adding fluoride to water.
The Medical Research Council said the chemical reduces dental decay but safety fears had to be addressed before more water companies add it to domestic supplies.
Dr Paul Harrison, director of the MRC Institute for Environment and Health, said: “There’s no reason to think that water fluoridation is responsible for any adverse health effects.
“But there is a lack of research on some important aspects, which is why we’re highlighting the need for more research.”
The research will compare how much of the element the body takes in where it occurs naturally and where it has been artificially added to drinking water.
“Because of the wide use of toothpastes and other dental health care products containing fluoride, we need a better understanding of how much fluoride we’re all absorbing,” Dr Harrison added.
“It is also important to know if there are any differences in the uptake of fluoride from natural and artificial sources.”
A review in 2000 confirmed the beneficial effects of water fluoridation on dental cavities.
But it also highlighted the association with dental fluorosis, a condition affecting the appearance of teeth.
The element has been added to the water supply of about five million people. Major schemes are in operation in Birmingham and throughout the West Midlands, and also in Tyneside.