Assessing the risk posed to aquatic organisms by the discharge of certain steroids（类固醇） and pharmaceutical products into waterways is often based on a belief that as the compounds degrade, the ecological risks naturally decline. But there's growing sentiment that once in the environment, some of these bioactive organic compounds may transform in a way that makes their presumed impact less certain.

A new study led by the University of Iowa and published online in the journal Science found this was the case with the anabolic（合成代谢的） steroid trenbolone acetate and two other drugs.

Once popular in the bodybuilding and weightlifting communities,trenbolone acetate is now banned for human use. However, it is federally approved for use by the beef industry to promote weight gain and increase feeding efficiency in cattle.

In lab tests followed by field experiments, the researchers found that trenbolone does not fully break down in water as believed, retaining enough of a chemical residue to regenerate itself in the environment under certain conditions, to an extent that the drugs' lives may be prolonged, even in trace amounts.

Researchers says the study is a first step toward better understanding the environmental role and impact of steroids and pharmaceutical products, all of which have been approved by the federal government for various uses and that have been shown to improve food availability, environmental sustainability and human health.

"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," says David Cwiertny, assistant professor in engineering at the University of Iowa and a co-corresponding author on the paper."What our work hopefully will do is help us better understand and assess the environmental fate of emerging contaminant classes. 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."

The team found similar results for dienogest, a hormone used in a birth-control pill called Natazia, and dienedone, a banned anabolic steroid that has been marketed as a body-building supplement.

Trenbolone acetate is implanted in the ears of more than 20 million cattle in the United States, according to studies cited by the researchers in their paper. The drug is metabolized and then excreted by livestock, and makes its way into waterways mainly through runoff.