Adding fluoride to the water supply prevents tooth decay, but women who drink fluoridated water during pregnancy may also trim the IQs of their male children by a few points, according to a Canadian study that suggests a serious drawback to a long-established public health intervention.

The study in JAMA Pediatrics looked at fluoride consumption by pregnant women and the effect on their babies by age 3 or 4. It did not examine whether drinking fluoridated water or getting the mineral from other sources after birth suppresses a child's intelligence.

The effect was clear for boys. Girls, on the other hand, showed an increase in I.Q., but it wasn't statistically significant, meaning it could have been due to chance. The gender difference may reflect the fact and male and female brains may develop differently, the researchers said.

For the typical mother-to-be living in a community that adds fluoride to the drinking water, the decline in IQ was 1.5 points (for boys only) or 2.3 points, depending on how fluoride exposure was measured.

"We're talking about the fetus and right now there is absolutely no benefit derived for the fetus" from fluoride, senior author Christine Till of York University in Toronto told Reuters Health in a telephone interview. "If anything, there is a potential for risk."

Thus, she said, the idea of limiting fluoride during pregnancy is "a no brainer" - and a major source is fluoridated water.

Fluoride only protects against cavities when applied directly to the tooth enamel, so there is no benefit to a baby until the child's teeth have appeared.

Based on the new results, and other research from Mexico City which found a similar decline in I.Q., "The hypothesis that fluoride is a neurodevelopmental toxicant must now be given serious consideration," said David Bellinger of Boston Children's Hospital in an editorial.

Adding fluoride to drinking water to promote dental health has been a fixture of municipal drinking water systems as far back as the 1950s. About two thirds of people in the U.S. now have tap water that is fluoridated, according to the Centers for Disease Control and Prevention. The rates are lower in other countries.

The mineral is also added to toothpaste, which most people spit out after use but children often swallow. Labeling beverages for fluoride is required in Canada but not in the U.S. and other countries.

Other studies have also suggested that fluoride poses this type of risk. Mexican research published in 2017 in found that fluoride consumption produced lower I.Q. scores at age 4 and in older children age 6 to 12. The Canadian study, involving 100 women and their children from 6 major cities, was an attempt to clarify the issue.

Dr. Bellinger said the I.Q. decline seen in the new study is comparable to what other tests have shown.

Study coauthor Angeles Martinez-Mier, a fluoridation expert and a professor at the Indiana University School of Dentistry in Indianapolis, said that although pregnant women should reduce their fluoride intake, for everyone else, "we don't have enough information to make policy recommendations, so you should stick with what you have," including leaving fluoridation in place.

She also said fluoride might come from different sources, depending on the region. In Mexico, the main source is fluoridated salt. In the U.S., in addition to fluoridated water, the mineral is found in tea, processed meats, sardines, grapes and raisins.

There's also the problem of cost. "Not all women have the means to pay for bottled water and that is a concern to me as a public health dentist," Dr. Martinez-Mier said.

If pregnant women decide to eliminate fluoridated water, "then they should take care of their teeth by reducing sugar and use other forms of topical fluorides that aren't ingested," Pamela Den Besten, a dentist at the University of California San Francisco, who was not involved in the research, told Reuters Health in an email.