What do you do with your old, expired medications? Flush them down the toilet, right? Out of sight, out of mind.

But you probably shouldn’t do that, as the chemicals will make their way into the local watershed. Which means you may eventually encounter them again—in stranger, and more disruptive, forms.

The problem is complicated. Even if everyone stopped flushing medicine down the drain, our own natural flushing, so to speak, will send those chemicals into the local watershed anyway. Sewage treatment plants can’t filter out things like endocrine disruptors, a class of synthetic chemicals that “either mimics or blocks hormones and disrupts the body’s normal functions,” according to the Natural Resources Defense Council.

“We are finding these chemicals all over the place,” Beth McGee, a senior water quality scientist with the Chesapeake Bay Foundation, told me last month. “It’s only been in the last 10 years where they have been looking for these chemicals. The next question is what they’re doing. What are the impacts at low levels?”

Seven years ago, male smallmouth bass in the Potomac River and its tributaries upriver from the District were discovered to have eggs in their testes, causing scientists to wonder if it was “the canary in the coal mine” to a much more serious problem. After all, the nation’s capital draws its drinking water from the Potomac, but there isn’t enough evidence to say for sure what’s exactly going on. There are theories, though, including sewage, livestock waste and pharmaceuticals like birth control pills that are thrown down the drain.

Despite the lack of conclusive evidence, D.C. Water advises customers to avoid flushing prescription medicine into the local sewage system. But there’s never been a coordinated public effort to curb drug-flushing habits.

So what can you do? On Sept. 25, the U.S. Drug Enforcement Agency is holding its first-ever “National Take-Back Initiative,” where people can turn-in prescription medications and other controlled substances to help curb their illegal use by those who are not supposed to have them. While the DEA is not pressing the water-quality concerns related to prescription medicine, the local turn-in events are a good way to keep drugs out of the wrong hands and out of the local watershed. The DEA will be incinerating the turned-in medication and won’t be flushing them down the drain.

Click here to find turn-in locations, including those at Metropolitan Police Department district stations.