Back in the 1990s, Theo Colborn, then-senior scientist with the World Wildlife Fund, sounded the first alarms about endocrine disrupters. In the book Our Stolen Future (Plume) Colborn describes her early findings that connected these endocrine disruptors – via chemicals in plastics, pesticides and pharmaceuticals – with male fish laying eggs and bald eagle eggs crumbling into tiny pieces. Soon, scientists developed new research techniques to study these estrogen-like compounds, which are highly active at trace levels. 


Now, those new research tools are putting the spotlight on an extremely persistent, and perhaps equally disruptive, group of contaminants: antidepressants. A new body of evidence is building. Study after study shows widely prescribed drugs such as Prozac, Effexor and Celexa disrupt the natural order when they are excreted into the water. Scientists in Mississippi discovered antidepressants are interfering with the way tadpoles develop into frogs. They also interfere with the ability of tiny minnows to escape predators. Experts say these early signs could point to long-term problems for the aquatic food chain as a whole.


Edward T. Furlong, Ph.D., a research chemist for the United States Geological Survey’s (USGS) National Water Quality laboratory in Denver, Colorado, says the reasons antidepressants wreak havoc on fish is because they work on the body’s serotonin system. Most organisms on Earth have this important neurotransmitter in their bodies, from the tiniest nematode (microorganisms in soil) to the largest mammals and humans. Once antidepressants disperse in the environment (in this case by traveling down streams in wastewater effluent), they can affect a wide range of living creatures. 


In fish, Furlong explains, serotonin is associated with aggression, predation and escape instincts. “The fish is in water continuously,” he says, “so dissolved antidepressants can cross the gills 24/7.” USGS scientists wanted to learn more about how these compounds – found in both water and sediment – might affect fish behavior. One 2010 study produced a surprising discovery: The antidepressants most common in stream water were not the ones that showed up in fish brains. 


So what happens when fish have antidepressants in their brains? Just like people, they mellow out. Some studies showed striped bass that uncharacteristically didn’t pursue smaller fish. Another important finding showed tiny fathead minnows that neglected to swim away when threatened by a simulated predator. 


Dana Kolpin, a researcher with the USGS Toxic Substances Hydrology Program, says that minnows usually react to predators with what is called a C-start mechanism. These fish didn’t. “It’s an innate behavior for fish. It’s how they escape predators,” he says. “They bend into a ‘C’ and escape with higher velocity.” 


The researchers are still trying to discover why this affected only the minnows in the larval stage, a very vulnerable part of their lifecycle. What’s clear is that this is not a good sign for the fish. 


Environmentalists like Renee Sharp, senior scientist with the Environmental Working Group (EWG), say this new research on fish and antidepressants points to the need for better testing. “It speaks to the need for more testing to look for ecological impacts,” Sharp says. “There are gaping holes in many aspects of our regulatory system.” 


U.S. Food and Drug Administration officials say they are keeping track of the latest USGS fish research, but don’t believe the levels of pharmaceuticals pose any risk to human health. 


MELISSA KNOPPER writes for E/The Environmental Magazine.