Miami-Dade’s mayor on Friday vetoed legislation that would strip fluoride from county drinking water, setting up a showdown override vote by county commissioners over a public health practice that began in the 1950s.

The veto by Mayor Daniella Levine Cava, a Democrat, pushed back on a growing campaign against the mineral, which has been used for decades to prevent cavities.

The change will take effect in 30 days, allowing residents to decide for themselves whether to consume fluoridated water.