PORTLAND, Oregon (AP) - The mayor of Portland, Oregon, has conceded defeat in an effort to add fluoride to the city's drinking water.

With more than 80 per cent of the expected ballots counted late on Tuesday night, the Multnomah County election website showed the fluoride proposal failing, 60 per cent to 40 per cent.

Rejection of the proposal would keep Portland the largest US city without fluoride in the water or plans to add it.

Portland's drinking water already contains naturally occurring fluoride, though not at levels considered to be effective at fighting cavities. Backers say adding more fluoride to the water is a safe, effective and affordable. Opponents describe fluoride as a chemical that will ruin the city's water supply, and violate an individual's right to consent to medication.