A new study may have you thinking twice about the ingredients in toothpaste – and your toothbrush.

University of Massachusetts researchers have found that triclosan, an antibacterial ingredient that has been banned by the Food and Drug Administration in soaps but is still allowed in toothpaste, accumulates in toothbrush bristles and is released into a user's mouth for about two weeks after they switch to a triclosan-free brand.

The study is the latest development in the continuing debate about the controversial chemical, which as of September is no longer allowed in over-the-counter antiseptic soaps, gels and wipes.

In short-term animal studies, high doses of triclosan were associated with a decrease in the levels of some thyroid hormones, according to the FDA.  But other studies have shown it is useful in dentistry. In 1997, the agency looked at the effectiveness data presented by the manufacturers of Colgate Total toothpaste, which showed that the chemical was effective in preventing gingivitis.

There is also concern the chemical can contribute to making bacteria resistant to antibiotics, according to the FDA. The chemical is a suspected endocrine disruptor linked to reproductive and developmental harm in laboratory studies, according to the Natural Resources Defense Council, which sued the FDA in an effort to have triclosan removed from soaps.

Colgate-Palmolive, the manufacturer of Colgate Total, which is 0.3 percent triclosan, said Wednesday that the chemical helps to "fight harmful plaque germs that can cause gingivitis, and it is approved as effective and safe by the U.S. FDA."

The study authors state that they do not consider oral exposure to triclosan toothpaste to be a health risk, the company noted.

The researchers were studying polymers – like nylon – and how liquids and gases were absorbed by the material or accumulated on the surface, said Jie Han, an environmental chemist at the Amherst campus who, along with Baoshan Xing, is a principal investigator of the study.

"We realized afterwards that the bristles on commercial toothbrushes are typically made of nylon. Then we started to look at the absorption of triclosan from toothpastes to toothbrush heads," she said.

The group tested six toothpastes that are commonly available in Massachusetts. Han declined to identify the actual brands of toothpaste used in the study.

They were able to determine that while the amount of triclosan did not exceed the amount consumers would get from one dose of the toothpaste, the exposure to the chemical continued even after they stopped using it, something that had not been known before, Han said.

"We are not studying the toxicity of triclosan," said Han. "We just wanted to present the facts of how much is absorbed from a toothbrush head."

The solution to the exposure would be to change toothbrushes if you change to a triclosan-free toothpaste, Han said.

"Customers can make up their own mind if they know this fact," said Han.