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Antidepressant Prozac In The Water Bizarrely Changes Guppy Fish Behavior

Johannes Van Zijl

Johannes Van Zijl

Johannes has a MSci in Neuroscience from King’s College London and serves as the Managing Director at IFLScience.

Managing Director

Guppy Fish

Fish housed in freshwater that had long-term exposure to fluoxetine had very limited variation in their behavior. Image Credit: fishex88/

Fish species around the globe are at risk due to the ever-increasing levels of pharmaceutical contamination that is making its way into water systems, according to collaborative research conducted by The University of Western Australia, Monash University, the Swedish University of Agricultural Sciences, and New York University.

Pharmaceutical drugs can make their way into water systems via urine and feces treated at sewage treatment plants. These plants rarely screen for drugs, and detectable levels of pharmaceutical contamination may remain in the water that is then released into streams and rivers by water outlets by them.


One such contaminant is the common antidepressant Prozac, generically known as fluoxetine.

Most people will have heard of the commonly prescribed selective serotonin reuptake inhibitor (SSRI) medication, used not only for depression but also for other conditions such as obsessive-compulsive disorder and bulimia. 

Now, a new study published in Proceeding of the Royal Society B: Biological Scienceshas revealed that fluoxetine can drastically alter the resilience of fish populations by altering their behavior and individualism to a major extent. These changes may make the fish more vulnerable to threats in their environment, and pose a real risk to their survival.  

“For fish populations to thrive in the face of environmental change, members of a group need to behave differently from each other,” Dr Giovanni Polverino, lead author of the study, said in a statement. “If a fish makes the wrong decision and dies, some others will survive by taking different actions."


“Unfortunately, we found that such behavioural diversity is eroded in fish populations exposed to fluoxetine, and might place large groups of fish at an increased risk of perishing in a changing and increasingly polluted world.”

To investigate the effects of fluoxetine, the researchers captured 3,600 guppy fish (Poecilia reticulata) from a local creek in northeastern Australia and raised them and their offspring over 2 years in freshwater tanks.

They split some of the fish into three sub-groups. One group of Guppies was housed in plain freshwater for the 2 year period, another group was housed in water that contained levels of fluoxetine commonly seen in the wild, the last group was housed in water that contained higher levels of fluoxetine, similar to those normally found near sewage plant outlets. The researchers repeatedly analyzed the activity and risk-taking behavior of the fish over time.

The findings of the new study revealed that fish that were housed in water without the drug fluoxetine had the biggest variation in their behavior over time – for example, it was clear that some fish were "lazy", while others were way more active and moved around often. Fish housed in freshwater that had long-term exposure to fluoxetine, on the other hand, had very limited variation in their behavior – they all kinda acted the same way even at relatively low-level exposure to fluoxetine. 


They basically become "zombies" as Science Magazine puts it. Guppies are known to have complex personalities, especially as it pertains to how they deal with stress and threats in their environment. These changes to their behavior shown in this study are of concern, as they may make these fish more susceptible to threads in the wild.   

Nevertheless, antidepressants save many human lives each year, but might come at a cost of their impact on the environment. The researchers state more work still needs to be done to better understand the impact of pharmaceutical contamination on fish, and how this work in the lab setting translates to what happens out there in the wild.

“Future research will help us understand how water contaminants influence individual variation in other traits which are ecologically important, such as metabolism, growth, number of offspring produced and ultimately, survival,” Dr Polverino concluded.


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