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.”

David Cwiertny, one of the study’s authors and a scientist at the University of Iowa’s Department of Civil and Environmental Engineering, told Healthline that much more work needs to be done. He said samples need to be taken from rivers and streams non-stop for 48 hours or more to truly understand the amount of trenbolone in the environment.

He added that the existence of trenbolone, which is sometimes taken illegally by bodybuilders and causes many confirmed health problems, likely is “far more persistent than all our models currently predict” in water near agricultural areas. “There’s a high degree of uncertainty in the existing occurrence data because of the trends we see,” Cwiertny said.

The FDA did not respond by deadline to a request for comment. The Environmental Protection Agency (EPA) declined comment.

But David Sedlak, a noted scientist who has mentored the study’s authors and has done previous work commissioned by the EPA, told Healthline that the research is “a wonderful piece of science and good detective work.”

Sedlak, co-director of the Berkeley Water Center at the University of California, Berkeley, said the results probably don’t indicate a threat to humans but may have repercussions for aquatic life. “It’s definitely a cautionary tale in the way in which measurements can be flawed if you don’t account for the reformation mechanism,” he said.

In the study, published in the journal Science, the authors call on pharmaceutical companies to come up with more eco-friendly steroids for cattle. Cwiertney said steroids provide undeniable benefits to cattle farmers and can even help reduce greenhouse gases.

Merck, a manufacturer of cattle steroids, provided this statement to Healthline: “This paper outlines an interesting observation related to the chemistry of (trenbolone acetate). The implications of these findings to the environment remain undefined and theoretical.”