Take a look at the bottle of antibacterial hand soap in your bathroom. Chances are good that a particular chemical is listed among its ingredients: triclosan.

The antibacterial substance, which was first developed in the 1960s to prevent bacterial infections in hospitals, has since been incorporated into everything from hand soaps to toothpastes to mouthwashes. Manufacturers see it as a marketing bonus, increasing consumer confidence that a particular product kills harmful bacteria. Even some household products—such as kitchen utensils, toys and bedding—include triclosan.

In recent years, though, research has shed light on a number of problems with employing triclosan so widely. Studies have shown that the chemical can disrupt the endocrine systems of several different animals, binding to receptor sites in the body, which prevents the thyroid hormone from functioning normally. Additionally, triclosan penetrates the skin and enters the bloodstream more easily than previously thought, and has turned up everywhere from aquatic environments to human breast milk in troubling quantities.

To this list of concerns, add one more: A new paper, published today in the Proceedings of the National Academy of Sciences, indicates that triclosan impairs muscle function in both animals and humans. The study, conducted by researchers from the University of California, Davis, found that the chemical hinders human muscle contractions at the cellular level and inhibits normal muscle functioning in both fish and mice.

“Triclosan is found in virtually everyone’s home and is pervasive in the environment,” said lead author Isaac Pessah. “These findings provide strong evidence that the chemical is of concern to both human and environmental health.”

In the first phase of the study, the researchers exposed individual human muscle cells, both from the heart and typical skeletal muscles, to concentrations of triclosan similar to what our bodies experience in everyday life. Then, they used electrical stimulation to cause the muscle cells to contract. Normally, electrical stimulations prompts an immediate muscle contraction—a mechanism that is responsible for the entirety of our muscle activity. In the isolated cells, though, exposure to triclosan disrupted communication between two proteins crucial for proper muscle functioning, causing failure in both the heart and skeletal muscle cells.

The research team also tested the effects of the chemical on two types of live animals—mice and fathead minnows. In the mice, heart muscle function was reduced by as much as 25 percent after exposure to a single dose of triclosan, and grip strength was reduced by as much as 18 percent.

The minnows were used in the experiment to mimic the effect of triclosan in marine environments. After being exposed to concentrations of triclosan equivalent to those found in the wild for 7 days, the minnows were significantly worse swimmers than minnows that hadn’t been exposed to triclosan, and were less effective in swimming tests that simulated the the act of evading a predator.

Using studies with animals to make assumptions about human health is always dicey, but the researchers say the fact that triclosan produced similar results in widely varying conditions with different animals—and the troubling effects of the chemical on human heart cells in test tubes—are causes for concern. ”The effects of triclosan on cardiac function were really dramatic,” said co-author Nipavan Chiamvimonvat. “Although triclosan is not regulated as a drug, this compound acts like a potent cardiac depressant in our models.” He speculates that in some cases, triclosan may be responsible for exacerbating heart problems in patients with an underlying condition.

Additionally, the FDA has declared that there is no evidence that using antibacterial soaps with triclosan confers any more health benefits than simply washing with conventional soap and water, and the agency is currently conducting a risk assessment for the chemical. ”Triclosan can be useful in some instances, however it has become a ubiquitous ‘value added’ marketing factor that actually could be more harmful than helpful,” said study co-author Bruce Hammock. “At the very least, our findings call for a dramatic reduction in its use.”