Scientists have found that the anabolic steroid trenbolone, thought to break down in sunlight, comes alive again at night.

They call it the “vampire steroid.”

Research released today shows that trenbolone, a steroid given to beef cows on industrial-scale farms, doesn’t break down in rivers and streams as previously thought.

The drug has been approved by the U.S. Food and Drug Administration (FDA) for use in cattle, but it is a schedule III controlled substance banned for use in humans. When treated cattle excrete feces, traces of the steroid end up in the surrounding environment, including local bodies of water.

Until recently, scientists believed that the compound, known to damage reproductive processes in fish, breaks down quickly in the presence of sunlight through a process called phototransformation.

Now, researchers know that when the sun goes down and the Ph level in the river is right, trenbolone reassembles. This means the amount of the chemical in water sources, which have likely been sampled and tested during the day, may actually be higher than previously believed.

Bryan Brooks, director of the Environmental Health Science program at Baylor University, who did not take part in the study, said the new findings raise important questions that need to be answered.

“Reports from this paper may stimulate rethinking the timing of environmental monitoring and surveillance,” Brooks told Healthline. “For example, the vast majority of routine water quality monitoring does not examine these unregulated contaminants. And if pharmaceuticals are examined in water bodies, sampling typically occurs during daylight hours and often only examines water samples from the surface of lakes and streams. Such a practice could over- or underestimate risks of various pharmaceuticals.”