Steroids given to livestock have a longer lifespan than expected, making their ecological impact less certain than previously thought.

A new study, published in the journal Science, found that the anabolic steroid trenbolone acetate and two other drugs that boost cattle growth do not fully break down in water as believed, leaving enough chemical residue for the drug to regenerate and live on.

"We're finding a chemical that is broadly utilized, to behave in a way that is different from all our existing regulatory and risk-assessment paradigms," study corresponding author David Cwiertny, an assistant professor in engineering at the University of Iowa, said in a university news release.

Lab tests and field experiments found that trenbolone, once a steroid of choice of bodybuilders but one that is now banned for human use, did not fully break down in water. While it disappeared in sunlight, in wet conditions, including one that’s mimicked water, up to 70 percent of the metabolites' initial mass regenerated.

"We knew something unique was going on," Cwiertny said. "In daylight, it essentially hides in another form, to evade analysis and detection, and then at nighttime it readily transforms back to a state that we can detect."

In the past, studies have shown that trenbolone is considered safe for the environment since it degrades quickly in sunlight in a process known as phototransformation. The latest study shows that when the sun sets and the acidic level in the water is right, the drug can regenerate itself and affect the fish population by reducing the amount of eggs produced and skewing the sex of some species.

“I’ve never seen anything like it,” Laura Vandenberg, an endocrinologist at the University of Massachusetts in Amherst, who was not involved with the study, told Nature. Normally water samples were taken during the day, when levels were low. But the study shows that without sunlight, the numbers rebounded -- in some instances it took five days for the chemical to regenerate 60 percent of itself.

While there hasn’t been visible repercussions of the “vampire steroid,” study authors say wildlife may still be at risk.

"We rarely see fish kills anymore, and we probably aren't discharging many carcinogens into surface waters anymore. But I don't believe this necessarily means that our water is safe for aquatic organisms," Edward Kolodziej, associate professor in engineering at the University of Nevada-Reno, said.

Cwiertny says the findings should shed light on other water contaminants.

“What our work hopefully will do is help us better understand and assess the environmental fate of emerging contaminant classes," Cwiertny explained. "There are a variety of bioactive pharmaceuticals and personal-care products that we know are present in trace amounts in our water supply. We should use what we're learning about trenbolone to more closely scrutinize the fate and better mitigate the impact of these products in the environment."