METHUEN — For the third time in 21 years, residents will vote on fluoridating the town’s water.

Town Council voted unanimously last night to attach a referendum to November’s state elections ballot, asking simply, “Do the inhabitants of the city of Methuen support the fluoridation of the Methuen water supply system?”

The town has answered no to that question twice before, in 1981 and 1997. Like both of those occasions, this referendum will technically be nonbinding. But Council Chairman William M. Manzi said he has assurances from both fluoridation opponents and the Board of Health — which legally has the final say — that they will abide by the results.

Residents could now get a full-fledged campaign as the pro- and anti-fluoride parties join gubernatorial candidates in vying for votes. Fluoridation opponent William Patenaude promised a repeat of 1997, when “we organized, gave out fliers, went door to door, just worked very hard, and spent our own money.”

Board of Health Chairman Dr. Dante Santone said that while the board won’t play a role, supporters have pledged their own public education effort.

Fluoride is endorsed by the world’s mental and dental establishments as a way to fight tooth decay, and most towns in the Merrimack Valley have been fluoridating the water for decades. But opponents insist it’s a form of mass medication that violates their freedom of choice and can cause health problems.

The issue resurfaced in Methuen last May, when the Board of Health voted to fluoridate without discussing it with town officials or opponents. Mayor Sharon M. Pollard responded by forcing Public Health Director Robert Katz to quit and replacing two of the three members on the board. The newcomers promptly rescinded the original vote.

But they joined Santone in asking the council to place fluoridation on this November’s ballot, agreeing that the public should have a say. Santone believes an increase in young families and a larger turnout for a state election could make the results different from 1981 and 1997.

“It’s a very different demographic, the voter turnout will be much higher — it may change the outlook,” he said. “And if it doesn’t, fine. It’s a matter of giving it a fair shot.”

Patenaude doesn’t think being fair is part of the equation and had hoped to at least put the referendum off for a year.

“We wish this vote never came up because it takes away our freedom of choice,” he said after the meeting, at which he waved a fistful of money he said would go toward fighting fluoridation. He estimates he spent $400 to $500 in 1997, and thousands crusading against it since then.

Manzi insisted the council has no choice but to hold a referendum since the Board of Health still has the power to implement fluoridation if members change their minds, and supporters have promised to press the issue.

“I didn’t bring this on, the Board of Health forced it on us,” said Manzi. “I don’t see any choice but to have the vote this year.”