Long-term exposure to triclosan, an antimicrobial agent commonly found in a broad array of soaps, shampoos, toothpastes and other consumer products, may have potentially serious health consequences, reports a research team including a UC Davis scientist.

Data from a new study shows that triclosan causes liver fibrosis and cancer in laboratory mice through molecular mechanisms that are also relevant for humans. The study, led by the University of California, San Diego School of Medicine, appears Nov. 17 in the journal Proceedings of the National Academy of Sciences.

“Triclosan’s increasing detection in environmental samples and its increasingly broad use in consumer products may overcome its moderate benefit and present a very real risk of liver toxicity for people, as it does in mice, particularly when combined with other compounds with similar action,” said Robert H. Tukey, a professor in UC San Diego’s departments of Chemistry & Biochemistry and Pharmacology.

Tukey led the study with Professor Bruce D. Hammock, who has a joint appointment in the Department of Entomology and Nematology, and Comprehensive Cancer Center at UC Davis. Tukey and Hammock are directors of National Institute of Environmental Health Sciences Superfund Programs at their respective campuses.

Tukey, Hammock and their research colleagues, including Mei-Fei Yueh of UC San Diego, found that triclosan disrupted liver integrity and compromised liver function in mice. The mice exposed to triclosan for six months (roughly equivalent to 18 human years) were more susceptible to chemical-induced liver tumors. Their tumors were also larger and more frequent than those in mice not exposed to triclosan.

The study suggests triclosan may do its damage by interfering with a protein called the constitutive androstane receptor, which is responsible for clearing foreign chemicals from the body. To compensate for this stress, liver cells proliferate and turn fibrotic over time. Repeated triclosan exposure and continued liver fibrosis eventually promote tumor formation.

Triclosan is perhaps the most widely used consumer antibacterial. Studies have found traces of it in 97 percent of breast milk samples from lactating women and in the urine of nearly 75 percent of people tested. Triclosan also is common in the environment: It is one of the seven most frequently detected compounds in streams across the United States.

“We could reduce most human and environmental exposures by eliminating uses of triclosan that are high volume, but of low benefit, such as inclusion in liquid hand soaps,” Hammock said. “Yet we could also, for now, retain uses shown to have health value — as in toothpaste, where the amount used is small.”

Triclosan is already under scrutiny by the FDA, thanks to its widespread use and recent reports that it can disrupt hormones and impair muscle contraction.  

Co-authors include Koji Taniguchi, Shujuan Chen and Michael Karin, UC San Diego; and Ronald M. Evans, Salk Institute for Biological Studies.

This research was funded, in part, by U.S. Public Health Service grants ES010337, GM086713, GM100481, A1043477, ES002710 and ES004699.