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Triclosan, a widely used antibacterial, is everywhere: in your cleaning supplies, soap, acne lotion, fabrics and even toothpaste. So too are the signs that it might be toxic: a 2012 study showed that triclosan impairs muscle contraction in cells, and other studies have linked it to endocrine disruption and bacterial resistance. Now, new research published Monday in Proceedings of the National Academy of Sciences finds that long-term use of triclosan may promote the growth of cancerous tumors in lab mice.

Mice exposed to triclosan for six months—which is the equivalent to about 18 human years—had significantly more liver fibrosis, or hardening of the tissues. “If you have a damaged cell that’s been attacked by a mutagen”—like tobacco smoke, for instance—”triclosan promotes the development of the tumor,” says co-leader of the study Robert H. Tukey, PhD, professor in the departments of chemistry and biochemistry and pharmacology at the University of California, San Diego School of Medicine. The compound also causes inflammation, which means that “all the ingredients necessary for developing cancer” are present, Tukey says. Compared to control mice, those exposed to triclosan grew tumors that were larger and more frequent. Triclosan may wreak such havoc by interfering with the protein that detoxifies chemicals in the body, the study says.

Researchers also found that in addition to causing liver fibrosis, triclosan caused some kidney fibrosis. That’s concerning to Tukey, since “there are really not a lot of environmental agents that have the potential to cause kidney fibrosis,” he says. “It definitely is doing some nasty stuff with long-term exposure in these mice.” Since it was performed in animals, the results don’t automatically apply to humans. But past studies have found the chemical in 75% of people and the breast milk of 97% of lactating women, suggesting at the very least that the chemical is ubiquitous.

“It has contaminated virtually all of the waterways in the United States, many in the world,” Tukey says. “It’s the major contaminant in sediment in most lakes. It’s present really everywhere.”

The FDA is “engaged in an ongoing scientific and regulatory review of this ingredient,” the agency website notes.

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