Demonstrators held a protest ahead of the decision
The Health Ombudsman says it may investigate a decision to add fluoride to tap water in Southampton.
Health bosses agreed plans in February despite 72% of 10,000 respondents in a public consultation opposing the move.
It was the first time a health trust in England, rather than water companies, was allowed to introduce fluoridation under new laws.
The ombudsman said it has received a number of complaints and could recommend the decision be overturned.
'Safe and effective'
A spokesman said it was looking into the complaints before decided whether to start an investigation.
He added 99% of the ombudsman's recommendations are implemented by health authorities.
The move to add fluoride was approved by the South Central Strategic Health Authority (SCSHA).
Opponents had voiced concerns over the impact it may have on people's health.
But the board said it was "satisfied that, based on existing research, water fluoridation is a safe and effective way to improve dental health".
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.