The anti-tooth decay chemical fluoride is to be added to the water supply across south Hampshire, potentially affecting 200,000 people.
They will join the 5.5 million people who drink similarly treated water across Britain, which experts say helps ensure filling-free teeth.
Southampton City PCT has said that it is the only way to tackle tooth decay in the city, where 40% of children have had a filling by the time they start school.
The South Central Strategic Health Authority took the decision under powers contained in the Water Act 2003, despite reported objections by seven out of 10 people in the area.
Meanwhile, anti-fluoride campaigners say that fluoride might be a risk to health, and that adding it to the water amounts to uncalled-for mass-medication.
They refer to reports that fluoride does little to stop tooth decay, although the West Midlands, which has used fluoride since 1964, reports that it has made a significant difference.
Copyright © Press Association 2009