You’d be excused for thinking that a plastic water bottle emblazoned with a “BPA-free” sticker is better for your health than a plastic bottle containing BPA. But know that it’s probably not. Next time, think about getting stainless steel instead.

At this point, many consumers are familiar with BPA, an estrogen-mimicking compound used to create epoxy resins and polycarbonate plastics. It’s found in everything from water bottles and soup cans to appliances and dental sealants. A series of studies over the past decade have hammered home the dangers of BPA, which has been linked to neurological problems, cancer, and obesity in various animals, among other things.

There are, of course, lots of toxic ingredients in the products we use every day, but BPA has happened to get an especially large amount of attention. As a result, manufacturers have made it a point to remove BPA from their products and let consumers know right on the label. But because BPA is an integral ingredient in resins and plastics, they had to replace it with something. In many cases, that something is a slight variation on BPA called BPS.

The bad news is that BPS is probably not much less toxic than BPA. We just don’t know as much about it because far fewer studies have been done on the replacement compound.

According to a new study published in the journal Environmental Health Perspectives, BPS has negative cardiac effects on rats–effects that are similar to what is seen with BPA. In the study, exposure to BPS (at levels similar to what humans are likely exposed to in daily life) caused female rats to develop an irregular heartbeat. “As a case study, our findings suggest that BPS and other structurally related BPA substitutes may share similar endocrine disrupting activities as BPA,” the researchers conclude.

This isn’t the first study to indicate that BPS is harmful. Another study, conducted in 2013, found that BPS has estrogen-mimicking properties. BPS has also been linked to hyperactive behavior.

Not every company that phased out BPA has replaced it with BPS, but if a product is simply labeled as being “BPA-free,” it’s hard to know. You can bet on this much, though: You’re exposed. A 2012 study from the New York State Department of Health found that 97% of urine samples taken from Albany residents contained the BPS compound.