"It's not a direct carcinogen," said study author Robert Tukey, a professor of chemistry, biochemistry and pharmacology at UCSD. "It's a tumor promoter."
In other words, exposure to triclosan encouraged existing liver tumors to grow. The mice who were exposed to triclosan had more tumors, bigger tumors and more frequent tumors than mice who weren't exposed to it, according to the study. The mice also developed liver problems, including scarring.
But experts not involved in the study cautioned that the mice were eating and drinking the triclosan in their food and water at "super high concentrations" for six months, which isn't comparable to using it for hand or hair washing.
U.C. professor Tukey said he and his colleagues fed the chemical to the mice to make sure they got an equal, standard dose for their experiment. He said it’s more triclosan than a human is normally exposed to, but it’s not yet clear whether low doses of the chemical would have the same tumor-promoting effect.
Esper said the study is a good first step, and that it shows that more research into how triclosan affects humans is needed.
Triclosan has been used since 1972, but last year, the U.S. Food and Drug Administration announced that it had no evidence to prove products containing it worked better than regular soap. Indeed, the FDA said some studies showed negative effects of using soaps with triclosan and triclocarban, such as "bacterial resistance and hormonal effects."
As a result, companies have until next winter to prove that soaps containing these chemicals are better than old-fashioned bar soap.
When it comes to the soap aisle, Esper said he recommends regular soap and good hand-washing techniques. The detergent in normal soap, he said, is enough to kill the germs without paying extra for soaps with added triclosan and other "antibacterial" chemicals.
Tukey said he doesn't want to be alarmist, but he won't use products containing triclosan.
"We don't see a little bit of tumors," he said. "We see very full blown tumorigenesis. It's on the extreme end of a tumor promoter and it does it very rapidly.”
The American Cleaning Institute, an industry trade group, said in a statement that the study does not prove triclosan promotes tumor growth in humans.
"The fact is that overdosing mice with triclosan at levels they would never likely come in contact with does not represent a realistic circumstance for humans," said Paul DeLeo, ACI associate vice president of environmental safety. "We've known for decades that the mouse is not a good model for human risk assessment of triclosan."