Fluoride will be added to tap water in Southampton after health bosses voted through the plans despite protests.
It is the first time a health trust in England, rather than water companies, has been allowed to introduce fluoridation under new laws.
The idea has proved controversial with 72% of 10,000 respondents in a public consultation opposing the plan.
Despite the opposition, the South Central Strategic Health Authority (SCSHA) unanimously backed the move.
Southampton City Council was in favour, but Hampshire County Council was against the plan, which is designed to cut tooth decay and which will affect 200,000 people.
Jim Easton, the SCSHA chief executive, said: "We recognise that water fluoridation is a contentious issue for some people.
"The board was satisfied that, based on existing research, water fluoridation is a safe and effective way to improve dental health."
I think it is absolutely disgraceful, they have refused to listen to all the evidence we have given them
John Spottiswoode, Hampshire Against Fluoridation
The SCSHA will now write to Southern Water instructing the firm to increase levels of fluoride from a natural 0.08 part per million to one part per million.
The Southampton City Primary Care Trust (PCT) said more than 40% of children in the area were suffering from tooth decay and the increase was desperately needed.
Bob Deans, chief executive for the PCT, said: "We are very pleased that following an extensive public consultation, SCSHA has decided to introduce a water fluoridation scheme."
The SCSHA said the problem of tooth decay among children and adults in Southampton was worse than the national average across England, despite a number of public health measures that have already been adopted in the city.
But opponents claim fluoride has negative effects on the body and vowed to contest the decision in the courts.
John Spottiswoode, chairman of Hampshire Against Fluoridation, said: "I think it is absolutely disgraceful, they have refused to listen to all the evidence we have given them.
"They have ignored the will of the people - 72% didn't want it and yet they still are going to do it. It is deeply unethical.
"We think it's illegal and are thinking what we do next, maybe taking it to the courts in Europe."
Health bosses in Southampton vote to add fluoride to the city's tap water despite protests.
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