(NewsNation) — Adding fluoride to water to prevent tooth decay may not be as effective as researchers thought decades ago, a new study published Friday suggests.

The Cochrane Library, an international not-for-profit organization, released the study to explore fluoride’s dental benefits and found it to have less efficacy than before 1975 when fluoride toothpaste became widely available.

Fluoride is a mineral that occurs naturally in water in small quantities, but water treatment plants may add additional fluoride. This is called community water fluoridation. Almost 73% of Americans drink water with fluoride added, according to the CDC.

The researchers analyzed 157 existing studies on the presence or absence of community water fluoridation in different places. They concluded that adding fluoride may lead to slightly less tooth decay in children’s baby teeth. However, there may be some benefit in that slightly more children do not have tooth decay.

A glaring limitation of the study is that it does not have evidence of the benefits for adults. Also, they could not find contemporary data on fluoride’s negative effects.

“There is insufficient evidence to determine the effect of cessation of CWF on (cavities),” the study said.

The presence of excessive fluoride can cause discoloration in the teeth, however. This is called dental fluorosis.

“If water contains 0.7 mg/L of fluoride, about 12% of people may have dental fluorosis that causes them to be bothered about how their teeth look, and about 40% of people may have dental fluorosis of any level. We were unsure whether fluoride in water leads to other unwanted effects,” the researchers found.

Since 1975, fluoride has been an ingredient in most toothpastes and is available in some mouth rinses, according to the study.

The Centers for Disease Control recommends a maximum of 0.7 milligrams of fluoride per liter of water.

The Department of Health and Human Services’s National Toxicology Program found in August that a higher level of fluoride exposure is linked to lower IQs in children. 

But, following that report, the American Academy of Pediatrics stood by its recommendation in favor of adding fluoride to water and toothpaste.

Despite this, late last month a federal judge ordered the Environmental Protection Agency (EPA) to address the potential impacts of fluoride in drinking water on IQ levels. 

Judge Edward Chen wrote that “a preponderance of the evidence” shows there is an “unreasonable risk” stemming from fluoride in drinking water.

In May, the Centers for Disease Control and Prevention said in a statement that “expert panels … have not found convincing scientific evidence linking community water fluoridation with any potential adverse health effect.”

Rachel Frazin with The Hill contributed to this report.