A decision to add fluoride to tap water in Southampton will go before the High Court later.
Resident Geraldine Milner began a legal challenge against the decision made in 2009 by the South Central Strategic Health Authority (SCSHA).
The SCSHA, which believes the move will improve dental health, gave the go-ahead despite a public consultation showing 72% opposed the idea.
The judicial review will decide if SCSHA properly considered the views.
During the consultation opponents had voiced concerns over the impact fluoride may have on people’s health.
But SCSHA said it was “satisfied that, based on existing research, water fluoridation is a safe and effective way to improve dental health”.
Stephen Peckham, chairman of Hampshire Against Fluoridation, said: “Local anger about the SCSHA’s decision has grown since 2009.
“People feel that fluoridation is being imposed on them without their consent or approval.
“If Ms Milner had not taken this action the SHA would have just steam-rolled ahead with a total disregard for the evidence and local opinion.”
Southampton City Primary Care Trust first proposed the move to increase the level of fluoride in water to one part per million.
Dr Jeyanthi John, consultant in dental public health for the trust, said: “Water fluoridation schemes have been in place in this country for some 40 odd years and shown to have proven benefits.
“About five-and-a-half million people in this country have been drinking fluoridated water.”
Professor Michael Lennon, who chairs the British Fluoridation Society, said: “Tooth decay remains in some parts of our country quite a substantial problem.
“If we take Southampton, in an average year more than 500 young children will be admitted to hospital for dental extractions… that is quite a traumatic experience.
“[If fluoride was added to the water] it would reduce tooth decay in children by about 40%, in adults by 30% and [we] would start to see a difference within three years.”