A nationwide shortage of fluoride has prompted eight communities northwest of Boston to scramble for the compound used to prevent cavities, and some water providers are likely to run out of the product soon.

In mid-August, a Florida manufacturer notified its distributor that it would no longer produce hydrofluosilicic acid, one of the products used to fluoridate drinking water. One of only a few such manufacturers in the country, US Agri-Chemicals’ plant shutdown in Fort Meade, Fla., is having an impact in this region.

The Massachusetts Water Resources Authority, which provides water to 47 communities, has found an alternative supplier, though at a higher cost. But Andover, Burlington, Lowell, North Andover, Reading, Tewksbury, and Winchester, which use smaller water providers, have felt the pinch.

Billerica ran out of fluoride for a week before making a purchase last month that will carry the town through early December, said John Sullivan, treatment chemist for the town’s Water Department.

While a short-term interruption of fluoridated water would have no significant effect, dentists say, communities are looking for ways to keep fluoride levels consistent so that residents don’t have to alter their dental care.

”As soon as we heard about the shortage, I called just about every chemical company out there, and basically, we’ve been told there is none left,” said Linda Hmurciak, assistant superintendent of North Andover’s water-treatment plant. ”The chemical companies who have some are only giving it to their regular customers.”

Hmurciak said she expected her remaining 200 gallons to run out by this week.

Lewis Zediana, who acts as purchasing agent for Northeast-Merrimack Valley Consortium of Water and Wastewater Facilities, said communities can buy fluoride on the spot market, where commodities are sold for immediate delivery. The consortium consists of 47 communities stretching from Sudbury to Gloucester. Sixteen of these communities use this type of fluoride, Zediana said, including eight northwest of Boston .

”What distributors are doing is providing to their contractual customers first,” said Zediana, the chief operating engineer at Tewksbury’s water-treatment plant, ”and then selling what’s left over.”

For the time being, communities will have to rely on the spot market to keep up their fluoride supplies. In the longer term, Zediana said, the other two manufacturers of hydrofluosilicic acid are expected to step up their production to meet the demand.

The consortium members and the MWRA were supplied by a distributor called Pennco Inc. of San Felipe, Texas. Its owner, Ron Horne, said he notified New England communities several weeks ago of the shortage, and is suffering a financial hit to his own company.

”It’s rough,” he said in a phone interview. ”We’ve probably lost 50 percent [of our] profit margin.”

In early October, Rita Convery, MWRA communications director, said the authority was able to secure a one-year contract with Houston-based Solvay Chemicals Inc. for its communities, which include Arlington, Bedford, Belmont, Lexington, Medford, Stoneham, and Winchester.

”It used to cost us $618 per ton, and we’re now up to $816 under this new contract,” she said. ”That’s why we locked in for only one year, because if the market changes, we don’t want to be paying $800 per ton a couple of years from now.”

Convery said the MWRA will pass on the additional cost to its customers.

Westford also is not affected by the shortage, said Robert Worthley, water-treatment manager, because the town uses a different compound, called sodium fluoride, which is not scarce.

”As a preventative measure, we purchased extra even though there wasn’t a shortage of the sodium fluoride,” said Worthley.

Of Massachusetts’ 351 communities, 135 add fluoride to drinking water, reaching 3.7 million people, according to the Massachusetts Coalition for Oral Health.

Residents of most communities in the region have fluoridated water, though Chelmsford, Methuen, Groton, Dunstable, and Wilmington are among those that don’t. The decisions not to fluoridate water were made by these communities’ boards of health, which are empowered by the state to decide the matter.

Many healthcare officials view fluoride as a health benefit, particularly for poorer communities where fluoride tablets are an expense and require a regimen, they say, that most parents would not indulge.

There are a lot of low-income communities where fluoride is a blessing in disguise, Hmurciak said. ”It’s something these kids would lack in their lives if not for this program,” she said.

Retired dentist Joseph G. Kalil, who practiced in Methuen for 40 years, is an outspoken advocate of fluoridation, and in 1995 participated in the 50th anniversary of the first community, Grand Rapids, Mich., to add it to the drinking water.

”The benefits were evident when I first started practicing in 1962,” he said. ”I could recognize patients from the communities they were coming from. For example, Andover was one of the first to fluoridate its drinking water, and I could recognize a patient when they came in for their initial examination. There were little or no cavities present.”

Kalil, a past president and current retired member of the Massachusetts Dental Society, unsuccessfully pushed for fluoridation of Methuen’s drinking water in 1981, 1997, and 2002.

”The fact is referendums do not resolve scientific questions,” he said. Still active in the American Dental Association, Kalil said he knows of no dentist who doesn’t support adding fluoride to drinking water.

Kalil said that if fluoride is missing for up to a year, there would be little or no effect on the incidence of cavities. But cavities would increase if the shortage persisted for several years, particularly among children with developing teeth.

Fluoride occurs naturally in some foods and water, but its levels in local water supplies is too low to protect against cavities, said Hmurciak. In addition to fluoridated water, people can get fluoride through rinses, toothpastes, gels, and tooth varnishes.

When the right amounts are absorbed by the body, fluoride is used effectively by the cells that make teeth and strengthen tooth enamel. Water officials in local communities say they add 1milligram per liter of fluoride to drinking water.

Too much fluoride can mottle the teeth. Because she feared parents would begin using supplements before the supply ran out, Hmurciak planned to announce the shortage in local newspapers this week, as she used her last 200 gallons.

”I just don’t want everybody to assume there’s no fluoride in the water, and then everybody will be over-fluoridating,” she said.

Five years ago, Wilmington’s director of public health, Gregory Erickson, recommended against fluoridating the town’s water, and the Board of Health concurred. Erickson characterized the current shortage as ”good news,” but acknowledged he’s one of the few area health officers who speaks against fluoridation of drinking water.

”It’s such an overwhelmingly accepted practice in spite of its problems,” said Erickson, referring to studies that cast doubt on fluoride’s dental benefits and suggest serious health risks.

After researching the issue, Erickson wrote in a 2000 recommendation to Wilmington Board of Health members, ”There is no compelling reason to fluoridate an entire community . . . with a toxic chemical at any dose, in the face of such compelling evidence against it. . . .”

Erickson recommended the use of fluoride tablets, which are prescribed by a dentist or physician. At the Andover CVS Pharmacy, 100 tablets containing the highest concentration of fluoride for older children and adults sells for $13.09. Prescriptions call for one tablet a day.