Traces of methamphetamine and other illegal drugs that enter waterways could cause addiction in fish, a novel study finds.

Recent laboratory experiments found that brown trout, a common fish in Eastern European rivers, exposed to methamphetamine at concentrations like those seen just downstream of wastewater treatment plants showed signs of addiction—such as being less active—and withdrawal. In the wild, meth-addicted fish could have difficulties reproducing and finding food.

“I was surprised that methamphetamine users can unknowingly cause fish meth addiction in the ecosystems around us,” Pavel Horký, a behavioral ecologist from the Czech University of Life Sciences in Prague, said by email.

Addiction to methamphetamine, a synthetic stimulant, is considered one of the most important global health threats, Horký