WORCESTER — While an often bitter controversy over fluoridation has once again been joined here, Worcester is not alone in struggling with what has become an increasingly charged public health issue.

Across the country and around the world, fluoride is generating public debate that is dividing communities and health officials in intense and often unpleasant disputes over whether the substance does more harm or more good — and whether local governments should force people to consume fluoride in their drinking water.

“I thought there was a lot of scare tactics being used on both sides,” said city administrator Michael Sigg of the clash over fluoride when it came before voters for the third time in Wooster, Ohio, last fall.

Voters in that relatively conservative Middle American city of 24,000 rejected fluoride by a few percentage points at the polls. Before the vote, they had been pushed and pulled by one group strongly in favor, made up largely of dentists, and an equally determined group of critics, he said.

“There were all sorts of claims that it was going to cause everything from osteoporosis to communism. For a lot of people, I think it came down to the fact that the scientists can’t agree, so let’s leave well enough alone.”

California proponents thought they had finally resolved the fluoride battle in 1996 when the Legislature, with the support of dental and health professionals, passed a state law requiring all water supplies serving more than 10,000 customers be fluoridated.

But instead of settling the issue the move triggered what some there are calling “The Fluoride Rebellion” — communities lining up to protest and, in some cases, defying the order.

The Santa Cruz Town Council was among the first to challenge the law, last year enacting its own law on a 6-1 vote to prohibit fluoridation of its water supply without a local majority vote. In March, voters passed a ballot question opposing fluoride.

Just last week, a nurse in Mountain View, Calif., began a signature drive aimed at reversing a decision made in the spring to begin fluoridating the water supply in that mid-sized Santa Clara County city. Among the studies the nurse cited was one performed by a community panel in Natick, Mass., to evaluate fluoridation, which “cites the harmful effects of fluoride.”

In Massachusetts, 135 communities now get fluoride in all or part of their water systems, and since the 1950s the fluoridation debate has bounced from one town and city to the next with mixed results.

As with the debate in Worcester, the controversy usually casts proponents, who argue that fluoride will have dramatic results in reducing tooth decay, with no ill effects, against critics, who point out it is a toxic substance and argue that it can actually damage teeth and poses risks of other ill effects from chronic exposure.

And here and elsewhere the debate often stings with name-calling on both sides. The Web site of the National Center for Fluoridation Policy and Research, for example, blames “small groups of anti-fluoride zealots” for stymying efforts to fluoridate water across the country. Fluoride opponents accuse proponents of arrogance and misuse of health studies.

The Center runs the results of the fluoride movement similar to sports box scores — cheering the approval of fluoridation in San Antonio; Las Vegas; Salt Lake City; Sunnyvale, Calif.; and North Attleboro last year, and chalking up another 3.9 million Americans who are drinking fluoridated water.

Meanwhile, it bemoans last year’s decisions to reject fluoride in 14 communities in the United States where a total of 366,347 people live. Those places include Spokane, Wash.; Brattleboro, Vt.; Ithaca, N.Y.; and Wooster, Ohio.

With scientific uncertainty prevailing in many debates here and in other countries, both sides point to the prestige of high-ranking public health officials and medical experts who have joined their side.

This year, the proponents in Worcester are citing the National Centers for Disease Controll and Prevention and two successive U.S. surgeons general endorsing fluoridation. Opponents, meanwhile, are stacking up last year’s co-winner of the Nobel Prize in medicine, Dr. Arvid Carlsson of Sweden, and the president of the Canadian Association for Dental Research, Dr. Hardy Limeback. They also cite a group of EPA scientists as experts recommending against fluoridation.

The debates can get bitter — so bitter in Winfield, Kan., last year that the City Commission decided not to put the community through the ordeal. After first deciding to put advisory referendums on the ballot there, the commission reversed itself. Commissioner Gregory Thompson said the issue would provoke more –name-calling, inflammatory statements and division” than helpful discussion. Instead, the commission voted to seek a grant to provide a new dental sealant program for underprivileged children to help reduce cavities.

Officials in other places have also sought alternatives to the fluoridation debate. Jamaica, for example, is experimenting with providing fluoride in special table salt.

Until recently, Ireland had been the poster child of the pro-fluoride movement. Since 1960, national law requires fluoridation of public water supplies in the Republic of Ireland. But a furor has erupted across the island since the county councilors of Kildare, following a lecture on fluoride from a British “expert,” agreed to have their urine tested and found that six of them had fluoride levels considered unsafe.

The ensuing debate over fluoride intake led to confirmation that Guinness beer, made from the public water supply in Dublin, had fluoride, forcing a dietary choice for the council chairman between his daily glasses of water and pints of Guinness to reduce fluoride levels in his body.

Since then there has been mounting pressure on Ireland’s minister of health to halt fluoridation, as councils in Dublin, Sligo, Donegal, Leitrim, Longford, Kildare, Clare and Kerry counties have stated their opposition to continued fluoridation without local approval. The Fine Gael Party there has made it a major plank in this year’s presidential elections, promising to end fluoridation if it wins.

Regardless of the debate on the fluoride issue, many millions of children across the United States and around the world suffer from terrible tooth decay problems. And virtually every study of that problem, including one completed last year in Massachusetts, concludes that the poorest children in communities both with and without fluoridation suffer the worst tooth decay problems.

In England, where few communities fluoridate public water supplies, the government last month started a new three-year project to provide 1 million children with free toothbrushes and toothpaste, and parents with brochures demonstrating correct brushing techniques. The program is being launched in 21 low-income areas with the highest level of tooth decay among children.

In Scotland, the Lankshire Board of Health has reported a similar pilot program, involving 10,000 nursery school children, that dramatically reduced the rate of tooth decay. While the results were gratifying to the local health officials, the study is being cited by anti-fluoride proponents just as the fluoride debate has erupted full force across Scotland.

Scottish Health Minister Susan Deacon has announced plans to begin fluoridating water across Scotland — and the Green Party has launched a national campaign to stop it. They argue that a £2.5 million program to provide free toothbrushes and toothpaste for toddlers would be more effective and less costly than the £4 million cost of running fluoridation plants.

Israel stumbled into the controversy this year, as well. The parliament last year voted to order fluoridation of every water supply in Israel. But this summer, the government announced a moratorium on the program until it completes a reassessment, prompted by the Israel Union for Environmental Defense, which has raised numerous questions about safety and the need for the plans.

But the largest problem involving fluoride in the world has little to do with the U.S.-styled water fluoridation programs. Rather, it centers on illnesses caused by dangerously high levels of fluoride that occur naturally in water supplies in some parts of the world.

According to UNICEF, the group that helps underprivileged children around the world, dangerous naturally occurring fluoride levels in water supplies have made fluorosis, or fluoride poisoning, endemic in at least 25 countries. The illness causes serious health problems for tens of millions of people, including large populations in Mexico, China, Pakistan and India.

The organization has launched a worldwide effort to detect and prevent fluoride poisoning in Third World countries. But UNICEF officials report millions of people are being exposed to levels of fluoride that lead to severe and permanent bone and joint deformations.

Such overexposure to fluoride, they report, causes a crippling condition of skeletal fluorosis. Early symptoms are head and stomach aches and muscle weakness. That leads to a further stage of osteosclerosis, a hardening and calcifying of the bones, and finally, irreversible damage to the spine, major joints, muscles and nervous system.

Accounts of villages in India and Africa where excessive fluoride levels have poisoned residents are disturbing. The Times of India reported last month, for example, that 70 percent of the 1,200 residents in one village are affected by rickets or arthritis because of high levels of fluoride.

Reports of similar problems throughout parts of India include widespread deformities, crippling and painful joint conditions. Health officials there report that fluorosis can lead to kidney failure and premature death as well.

Concern about excessive fluoride in water is also not unusual in the United States, and health officials continue to stress the need to constantly monitor and carefully control fluoride levels in drinking water.

Last month, for example, the U.S. Environmental Protection Agency warned that children under age 9 in Canyon, Texas, should stop drinking local water. An annual test had found that natural levels of fluoride in their drinking water were 50 percent higher than the level the EPA considers safe.