An antibacterial chemical found in thousands of consumer products can harm the guts of mice, according to a new study, raising concerns that it might be making people sick.Triclosan is a popular additive in many consumer products because it kills bacteria. Since the 1950s, it’s been added to hand sanitizers, toothpastes, cookware, gardening tools, clothes, toys, furniture, and even some baby teething products.In the last decade, the chemical has permeated the broader environment as well in tiny amounts. It’s been found in household dust and in US streams and rivers. It’s also in people: By 2003, an estimated 75% of the US population carried it around in detectable amounts, according to the CDC.A raft of studies has suggested that triclosan has unintended effects beyond its antimicrobial properties. For instance, it can make some bacteria grow stronger and harder to kill. Scientists suspect it may also tamper with the immune system, certain hormones, and fertility.In 2016, the FDA banned triclosan in hand and body washes, explaining that there was not enough evidence that triclosan was safe to use, or that washing with regular soap and water was any worse.The new work, published Wednesday in the journal Science Translational Medicine, shows how the chemical can wreak havoc with the community of bacteria — the “microbiome” — found in the mouse gut, just as the chemical changes the microbiome of fish and rats.Mice were fed water containing triclosan for three weeks, then compared to a group that hadn’t eaten the chemical. The goal was to mirror in mice the blood triclosan levels of people who had used toothpaste containing the chemical for 14 days.In mice that drank the triclosan-laced water, the scientists found inflammation of the colon akin to that seen in inflammatory bowel disease (IBD), a condition diagnosed in 3 million Americans in 2015. They also showed that mice that already had colon cancer showed a more aggressive form of the disease — with more and larger tumors — when they consumed triclosan.Gut bacteria, the study found, were key to activating these effects: Mice bred to lack gut bacteria didn’t show any inflammation even when they were exposed to triclosan.“We have very strong data to confirm that gut bacteria is the mechanism to link triclosan exposure to colon inflammation,” Guodong Zhang, assistant professor at the University of Massachusetts, Amherst, who led the study, told BuzzFeed News.The new paper adds to mounting evidence that triclosan is not safe. Although it’s too soon to know whether it has the same effects on the human gut, the study makes the case for examining its link to IBD and colon cancer in people.”I think it’s an opportunity to point out that people should be careful,” Rolf Halden, director of Arizona State University’s Biodesign Center for Environmental Health Engineering, who was not involved with the study, told BuzzFeed News. Last year, Halden led a group of 200 scientists to publish a statement asking international regulators to ban the chemical in consumer products.The latest study caps off decades of research listing possible dangers of triclosan exposure in animals and people. Scientists have shown it can cause liver damage in mice, for example, and mess with hormones in rats, and alter the microbiomes of fish.It’s been harder to pinpoint whether triclosan at low doses can cause harm in people. Scientists expect it can interact with many biological systems in complex ways, possibly over many years, making its footprint hard to map. Last year, researchers followed 39 new mothers and their infants for 10 months and found higher levels of an antibacterial-resistant species in the gut of participants who used products with triclosan.Regulation of the chemical falls to both the FDA and the EPA. (The EPA regulates the compound when it is used as a germ killer on furniture or products that don’t make health claims relevant to people.)The FDA proposed banning the chemical in consumer products as early as 1978 but never finalized that rule. In 2010, the National Resources Defense Council (NRDC) sued the FDA to move forward with its intended action, resulting in a settlement, and the issuance of the first of its rules in 2016.“Triclosan is interacting with the microbiome in a way that is leading to inflammatory bowel disease,” Kristi Pullen Fedinick, a scientist at the NRDC told BuzzFeed News. “They’ve really gotten close to if not landed on the causal link.” She described the new evidence on colon cancer as “the tip of the iceberg.”The FDA’s ban to excise the chemical from hospital antiseptics will take effect at the end of the year, and the agency intends to ban it in hand sanitizers by 2019.Some doctors already recommend avoiding products containing the compound.“Both the FDA ban and this new study indicate that consumers best steer clear of unnecessary and potentially risky antimicrobials unless they are specifically recommended for use by their physicians and dentists,” Halden said.