People in West Cumbria will have fluoride added to their drinking water in the coming weeks after a seven-year break.
NHS North West has confirmed that water could start having fluoride added again next month, although a firm date has not been set.
Health experts say that adding fluoride to water supplies helps to reduce tooth decay.
A spokesman said the healthy authority had a legal duty to provide fluoridated water.
In 2006 the process of adding fluoride to water ended at the Cornhow water treatment works at Loweswater because United Utilities, which runs the plant, needed to upgrade equipment.
That work is nearing completion and the spokesman said fluoridation could begin in April, although it could be later depending on progress.
The chemical will be added to water at a rate of one part fluoride to one million parts of water.
The spokesman said four reviews over the past 12 years proved that fluoride in drinking water provided dental health benefits.
But some West Cumbrian residents fear that treating water with the substance, mainly produced as a by-product of the fertiliser industry, could cause hidden health damage.
Father-of-four Dave Alty, of Redmain, near Cockermouth, has researched the issue after finding out the process was due to start.
He said: “For every one of those studies there are 50 that are saying there are unhealthy aspects to fluoridating the water.
“There’s fluoride in toothpaste. Why do we need to contaminate the water? It’s banned in most of Europe.”
The spokesman said the service would conform to all European standards for water treatment chemicals and water supply regulations.
He added: “Plans are being put in place to ensure that members of the public in the locations affected will be fully informed once we have a firm date for when the service will resume.”
The Cornhow works supply water to most of Workington, Seaton, High Harrington and Great Clifton, the coastal areas from Silloth, through Maryport, to Flimby, and inland to Cockermouth.
Salterbeck gets its water from Ennerdale, along with much of Copeland, and its water has remained fluoridated during the break in supply at Cornhow.
A United Utilities spokesman said: “Like other UK water companies, United Utilities has a duty to fluoridate water supplies if requested to do so by the strategic health authority.
“We do not make or influence these decisions.”