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Sometimes it feels like science is always changing its mind about what’s good for us: Obesity is a killer unless, as new research suggests, it’s helping you survive infection; and caffeine is bad for you, except of course if it’s not.

But pollution—surely, pollution!—can’t possibly be good for any living thing. Then again, in some particular forms, maybe it is. At least that’s the implication of a new study published in Environmental Research Letter that looked at how an environmental contaminant can boost the life prospects of a kind of fish.

To be fair, we’re not talking about oil spills or mercury poisoning here. Instead, the researchers, from Umea University in Sweden and led by ecologist Jonathan Klaminder, looked at the effects of the drug Oxazepam on Eurasian perch, a common lake fish. Oxazepam is an anti-anxiety medication that has been widely prescribed in human populations since the 1960s, and is now found in relatively high concentrations in many lakes and rivers. The contamination mechanism is straightforward: Patients take the drug and pass it into the water supply through their urine.

Anti-anxiety drugs and fish are an amusing combination, and the researchers wanted to determine whether the drugs benefit the perch in at all the same ways they help people. They collected 120 fish from a lake in southwest Sweden and split them into three groups, each of which was placed in a tank with different doses of Oxazepam. They did the same thing with perch roe—to see whether drug effects might take hold before birth—and afterward compared how fish in the different groups behaved. The effects were striking: Fish with high exposures to the drug—included newly hatched ones—became bolder, more active, and less social than fish that hadn’t been medicated. This made them “more efficient foragers,” and less likely to die in their tanks.

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The results aren’t exactly a call to start dumping yet more prescription drugs into our water supply. The researchers do think, however, that their study highlights a kind of bias in the way ecologists study pollution: Tests are so geared to find negative effects that they miss any positive effects—like a hefty, appetitive perch swimming around with a perfect pharmacological buzz.

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Kevin Hartnett is a writer in South Carolina. He can be reached at kshartnett18@gmail.com.