PORTLAND — The City Council approved a plan Wednesday to add fluoride to Portland’s water, meaning Oregon’s biggest city is no longer the largest holdout in the U.S.

The ordinance calls for city water to be fluoridated by March 2014.

Dental experts say fluoride is effective in fighting cavities. Opponents of public fluoridation say it’s unsafe and violates an individual’s right to consent to medication.

“Reasonable people can disagree, but the science is on the side of fluoridation,” Mayor Sam Adams said after the unanimous vote.

Opponents also say council members rushed into action without a public vote. They plan to collect almost 20,000 signatures in the next 30 days to force a referendum early next year — before the mineral is added to a water supply that serves about 900,000 people in Portland and a few suburbs.

“There is no question that we are going to need a lot of financial and volunteer support to make this happen, but we are seeing a major backlash to how the City Council has handled this,” said Kim Kaminski, executive director of Oregon Citizens for Safe Drinking Water.

Public fluoridation remains an emotional topic in many parts of the country. The issue arose in Phoenix this week when a public stir prompted re-examination of a policy in place since 1989. After a contentious hearing Tuesday, council members voted to continue adding fluoride to the water in the nation’s sixth-largest city.

Wichita, Kan., residents will vote in November whether to add fluoride to the city’s water supply.

Portland’s drinking water already contains naturally occurring fluoride, though not at levels considered to be effective at fighting cavities.

Voters in Portland twice rejected fluoridation before approving it in 1978. That plan was overturned before any fluoride was ever added to the water. Before announcing his vote, Commissioner Randy Leonard said the passion from both sides showed why previous councils sidestepped the matter: “This issue is not for the faint of heart.”

Dozens of opponents turned out for the vote that went as expected. They booed, hissed and held signs that said “Public water, public vote.” When it became apparent that commissioners would indeed approve the ordinance, they stood with their backs turned.

Regina LaRocca, who had her back turned, said the “money and power” on the side of fluoridation was difficult to overcome, but commissioners might have altered their votes had more people flooded City Hall.

“I’m disappointed in the people of Portland, and their apathy,” she said.

Last week, 227 people — most of them opponents — signed up testify at a public hearing that last 6 1/2 hours.

Commissioners said now is the time to act because Portland children have more dental problems than kids from neighboring states that fluoridate, and adding the mineral to the water is the most safe, effective and affordable way to address it.

Seventy-three percent of the U.S. population drinks water treated with fluoride — more than three times the rate in Oregon.

Fluoride opponents contend the dental benefits of fluoride are small and don’t outweigh the cons. Several pointed to a recent study by researchers from the Harvard School of Public Health, which appeared in Environmental Health Perspectives. It showed that Chinese children who had high levels of naturally occurring fluoride in their drinking water had lower IQs than those who lived in villages with less fluoride in their water.

“These people are putting their children’s teeth above their children’s brains, and that’s ridiculous,” said Paul Connett, author of the book “The Case Against Fluoride,” who traveled to the meeting from New York.

Residents with thyroid issues, kidney disease and multiple chemical sensitivity worry fluoridation will make their lives worse. Others are concerned it can cause cancer, autism and other problems.

Commissioners counter that more than 200 million Americans drink water with added fluoride, and it doesn’t appear to have caused great harm. Moreover, it is endorsed by most health organizations, including the American Medical Association and Centers for Disease Control.

Fluoride supporters and opponents agreed that fluoridation alone won’t solve Portland’s dental problems. They say parents must stress proper hygiene and stop giving their children drinks loaded with sugar.

But the commissioners say fluoridation will at least give children, particularly those from families without money or education, some protection against tooth decay.

“A 3-year-old child can’t be expected to take responsibility for their dental care,” Commissioner Dan Saltzman said, to which a frustrated member of the audience shouted: “Their parents can.”