A pharmaceutical pollutant, clobazam, is appearing in global waterways and altering the behavior of marine creatures, according to new research on psychoactive drugs and their environmental impact.
Pharmaceutical pollution is an increasing threat to biodiversity, ecosystem functioning, and public health that remains poorly understood. Regions as remote as Antarctica have been affected, as over 900 pharmaceutical or pharmaceutical-derived compounds have been detected in waterbodies globally.
Pharmaceutical Pollutant Effects
Problematic for the ecosystem, these drugs are designed to be effective even in very low concentrations, and are highly persistent in the environment. While created for use by humans, research shows that drugs like antidepressants and anxiolytics also affect animal behaviors, even overriding survival mechanisms. Until now, most studies observing the behavioral effects of pharmaceuticals on animals have focused on laboratory work while ignoring the broader ecological impact of pharmaceutical pollution, which includes plant life.
“While our research focuses mainly on how pharmaceutical pollution affects animal behaviour and survival—especially in fish—other studies have indeed detected traces of these drugs in plant life,” lead author Jack Brand, of the Swedish University of Agricultural Sciences, explained to The Debrief. “What’s more, pharmaceuticals can move through the food web, sometimes even becoming more concentrated as they are transferred up the food chain.”
“This can affect not just aquatic life but also land animals that feed on insects or fish from contaminated waterways, showing how far-reaching these effects might be.”
Field Work In Europe
Jack Brand led the team through years of field experiments investigating the impact of psychoactive pharmaceuticals on Atlantic salmon behavior.
“We carried out our fieldwork on the River Dal (Swedish: Dalälven) in Central Sweden. Specifically, we tracked young Atlantic salmon as they migrated nearly 30 kilometres downstream toward the Baltic Sea—an important and challenging journey in their life cycle,” Brand said.
The researchers found that groups of young salmon called smolts were more likely to reach the sea after exposure, which the team hypothesizes is due to the salmon becoming less risk-averse and minimizing their normal grouping behavior, called shoaling. Overall migration speed remained relatively consistent, but exposed salmon traveled through hydropower dams more quickly, likely due to taking riskier navigational strategies.
Laboratory experiments also supported the reduced shoaling, which could have the negative consequence of opening salmon to increased predation. The pollution’s end effects are complex, aiding migration yet increasing vulnerability.
A Psychoactive Drug Enters Our Waterways
The main culprit was the anxiolytic drug clobazam, which the team discovered accumulating in salmon’s brains. One of the more common pharmaceutical pollutants, the drug continues to enter natural waterways.
“Clobazam and other pharmaceuticals enter the environment in a few different ways—mainly through production, use, and disposal,” Brand explained. “When people take medications like clobazam, their bodies often don’t absorb the full dose. What isn’t absorbed is flushed away and ends up in wastewater.
“Unfortunately, most wastewater treatment plants don’t fully remove these substances, so a portion of these drugs make their way into rivers, lakes, and streams, where they can persist and impact wildlife,” Brand added.
“Our findings raise important questions about how pharmaceutical pollution alters migration behavior and survival in the wild,” said Brand.
Understanding the Pharmaceutical Pollutant Problem
These drugs accumulate in species that humans consume, creating an even more direct cause for concern. Several recent studies have identified drugs in seafood, although most were minute traces unlikely to affect humans. However, in a handful of incidents, the amounts discovered were found at levels that were potentially dangerous to infants or even adults consuming a diet high in seafood. Continuing research is necessary to grasp how serious the danger may be.
Going forward, Brand’s team aims to tag fish with small electronic tags to measure their stress levels and survival rates, identifying the long-term effects of pharmaceutical pollutants.
“Since writing the paper, we’ve been developing follow-up studies to look at how pharmaceutical exposure affects survival once salmon reach the sea—a question that’s especially important given their long and risky ocean journeys,” Brand said. “We’re also continuing to investigate how other common drugs influence fish behaviour and ecosystems.”
“More broadly, our findings show that even tiny traces of human medicines can have wide-reaching effects on wildlife and ecosystems.”
The paper “Pharmaceutical Pollution Influences River-to-sea Migration in Atlantic Salmon (Salmo salar)” appeared on April 10, 2025, in Science.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.