People in Southampton and west Hampshire have 14 weeks to make their voices heard over plans to add fluoride to tap water.
Health bosses say more than 40% of children in the area are suffering from tooth decay and believe the move would reduce the problem.
But campaigners said studies had shown fluoride can have a detrimental effect on the body and does not fight decay.
If the plans are approved about 200,000 people would be affected.
John Spottiswoode, chairman of Hampshire Against Fluoridation, said: “It is totally wrong to force people to drink water with a toxin in.
“There’s lots of research that shows a very slight benefit for children but not adults.
- “We’ve tried a number of ways to change people’s behaviour in the past and that has not achieved the sort of changes we want.”
Dr Jeyanthi John, PCT consultant in dental public health
“It’s been shown if you drink the water with fluoride in it, it can have a damaging effect on the body and has been linked to health problems.
“It has also been shown to cause 48% of people in fluoridated areas to get dental fluorosis.
“This means they get flecks of white in their teeth and they can became brown and it is very expensive to treat.”
Southampton City Primary Care Trust (PCT), which wants the level of fluoride increased to one part per million, said it was desperately needed.
It said more than 520 children in the city needed a general anaesthetic to have a total of 2,900 teeth extracted last year.
Dr Jeyanthi John, consultant in dental public health at the PCT, said: “We’ve tried a number of ways to change people’s behaviour in the past and that has not achieved the sort of changes we want.
“This is a golden chance for Southampton’s children and adults to join the 5.5 million people in the UK and 350 million people worldwide currently benefiting from safe and effective fluoridation schemes.
“I would urge everyone to support this public health measure.”
The 14-week public consultation is being independently-run by the South Central Strategic Health Authority.