Boys who drink fluoridated water are five times more likely to develop a rare but deadly form of bone cancer than boys who drink water without the chemical, Harvard University researchers have found.

The finding, if verified by further research, would disprove the U.S. government’s long-held contention that adding fluoride to tap water prevents tooth decay without causing dangerous illnesses.

Researchers found a link between fluoridated water and osteosarcoma for boys ages 4 to 12, and said it is biologically plausible that fluoride would affect bones the most during periods of childhood growth.

The cancer link was strongest in 7-year-old boys, whose risk of developing bone cancer increased fivefold if they lived in fluoridated communities.

“The study raises very serious concerns about fluoride’s safety and its potential to cause bone cancer” Environmental Working Group senior vice president Richard Wiles said in a news release. “The findings raise fundamental questions about the wisdom of adding fluoride to tap water.”

Researchers found no link between fluoride and bone cancer in girls.

Fluoride is added to drinking water in 137 Massachusetts cities and towns, including Framingham, Marlborough, Natick, Hudson, Wayland, Holliston, Newton, Waltham, Sudbury, Wellesley, and Dedham.

The Harvard researchers, led by oral health policy and epidemiology instructor Elise Bassin, published their findings this week in Cancer Causes and Control, the official journal of the Harvard Center for Cancer Prevention.

It was the first time Bassin’s findings were published in a peer-reviewed journal, though she initially detailed the bone cancer link in a 2001 doctorate dissertation.

The dissertation became controversial because Dr. Chester Douglass, Bassin’s doctoral thesis adviser at the Harvard Dental School, is under investigation by Harvard for allegedly misrepresenting her work in a report to the National Institute of Environmental Health Sciences.

Douglass, who is on the payroll of fluoride toothpaste maker Colgate and received federal grants to study bone cancer and fluoride, may have “lied about the results of (Bassin’s) work” when he reported finding no link between fluoride and bone cancer, according to the Environmental Working Group, which filed the ethics complaint against him.

Natick chemist and fluoride researcher Myron Coplan applauded Bassin for getting her work into the public eye. “She hasn’t backed off one bit from the thesis,” he said.

Bassin’s analysis, based on 103 cases of osteosarcoma and a control group of 215 people, is supported by similar findings in two previous epidemiologic studies and a report linking fluoride to bone cancer in male rats.

But “further research is required to confirm or refute this observation,” Bassin wrote.

The Centers for Disease Control and Prevention said it is examining the study.

“It’s a fairly complex article. We’re reviewing it,” said CDC health communications specialist Linda Orgain.

But the CDC won’t change its stance on fluoride unless the “weight of the evidence” shows it is dangerous, she said.

“Our 60 years of experience (with water fluoridation) shows it’s effective for preventing decay and safe,” Orgain said.

Last month, the government’s National Research Council urged the Environmental Protection Agency to lower the maximum allowable amount of fluoride in water to prevent bone fractures and tooth damage.

Fluoride added to water for dental reasons is set at 1 part per million, while the highest allowed amount is 4 parts per million.

A member of the NRC committee said it’s “hard to rely on any one study” but said Bassin’s examination of cancer risk by age was interesting.

“It’s an interesting idea, looking by age,” said professor Thomas Webster of the Boston University School of Public Health. “It’s possible, but it’s sort of exploratory.”