Benzodiazepines are a highly effective group of pharmaceutical drugs that help millions of people cope with anxiety, insomnia, and panic disorders each year. These drugs work by binding to receptors in the brain and enhancing the effect of a neurotransmitter called GABA.

But humans aren’t the only animals with GABA and GABA receptors; several other species—including many types of fish—are similar to us in this aspect of their brain chemistry. And, thanks to the huge amount of prescription drugs that are flushed down the drain (American medical care facilities alone may flush as much as 250 million pounds of pharmaceuticals a year), these species are inadvertently being exposed to high concentrations of benzodiazepine in rivers, lakes, and streams.

In this week’s issue of Science, a group of researchers shows exposure to these drugs can cause significant behavioral changes in fish, changes that could potentially alter ecosystem dynamics.

Working in Sweden, the scientists found that treated wastewater had concentrations of oxazepam, a common benzodiazepine, of 0.73 µg per liter. The oxazepam concentration in a nearby stream where treated wastewater was released wasn’t much lower, at 0.58 µg per liter. Moreover, they found that European perch (Perca fluviatilis) that live in the water were actually accumulating the drug in their bodies; the oxazepam concentration in their tissue was more than six times that of the water.

To figure out whether drug exposure could be affecting the behavior of these fish, the researchers exposed perch to water with varying concentrations of oxazepam over seven days. The lowest experimental concentration was “environmentally-relevant,” representing conditions that perch likely experience in streams and rivers, while the highest concentration was more than 500 times higher.

Suspecting the drug could alter the behavior of the fish, the researchers quantified each perch in terms of several “personality” traits, both before and after benzodiazepine exposure. They examined three different tendencies—boldness, activity, and sociality—that are generally consistent within individuals and are often important to the animals’ fitness. Boldness was measured by testing how long it took each fish to explore a new environment; activity was quantified by the number of swimming bouts during the trial period; and sociality was a measure of how close the fish stayed to others.

Even small amounts of benzodiazepine affected the fishes' behavior. At the lowest concentration, oxazepam made the perch more active and less social; at higher levels of exposure, boldness increased as well.

As a better proxy for fitness-related behavior, the researchers looked at whether benzodiazepine exposure affected the feeding rate of the perch. Before exposure, it took a single fish more than 4½ minutes to consume 20 zooplankton. After low levels of exposure to oxazepam, the fish ate faster, shaving about 30 seconds off the pre-exposure time. Fish exposed to the highest level of oxazepam consumed the same amount of zooplankton in less than three minutes. Not only were the exposed fish feeding faster; they also started eating earlier in the trial than they had before being exposed to the drug.

These behavioral changes are likely to have significant effects on benzodiazepine-laced food webs, although it’s unclear exactly what the implications will be. The first possibility is that affected fish will feed so quickly that they will decimate the zooplankton population, causing a spike in algae (which zooplankton generally keep under control). Neither zooplankton nor algae have GABA receptors, so these species would only be affected indirectly by the presence of benzodiazepine in the water. Alternatively, overly-bold fish could bring on their own demise by taking unnecessary risks and leaving themselves open to predation.

The effects of pharmaceuticals on delicate food webs are unpredictable, but one thing is certain: whatever the consequences of benzodiazepine exposure will be, they will likely be irreversible if we can’t keep this drug out of the water.

Science, 2013. DOI: 10.1126/science.1226850 (About DOIs).