It is a simple act, having a prescription written in a clinic. Just like it is an ordinary one to take a small pill taken with water. Now imagine that same molecule, weeks later, traveling far beyond that moment of care. Not disappearing and not fully breaking down, but moving through a human body, into wastewater, through treatment systems never designed to catch it completely, and then out into coastal waters where tides blur the boundary between city and sea.
In Rio de Janeiro, that very scenario is playing out on their coastline. And beneath the surface of those turquoise waters, marine life continues on without knowing their life is colliding in detrimental-to-them ways. Recent findings first reported on The Conversation from the EcoShark Project (as the research is “to be submitted for publication, but already shared among UFRJ scientists”), coordinated by researchers at the Carlos Chagas Filho Institute of Biophysics at UFRJ found that the above scenario most likely explains why scientists detected sertraline, one of the most widely prescribed antidepressants in the world, in shark tissue collected off Rio’s coast. In some cases, traces were found in brain tissue of hammerhead sharks (Sphyrna lewini and S. zygaena), including species already classified as critically endangered.
We often think of pharmaceuticals as something contained within hospital systems and households. But molecules move and systems overlap. Waste becomes part of water, and water intertwines with habitat.
So how exactly does a drug designed to alter serotonin levels in the human brain end up in a shark’s nervous system? The path is more direct than it first appears: after ingestion, sertraline is metabolized in the liver, but not completely as a portion is excreted unchanged or as active metabolites. These compounds then enter sewage systems, many of which are not equipped to remove pharmaceutical residues. In Rio de Janeiro, estimates from Brazil’s National Sanitation Information System suggest that not even half (47%) of wastewater receives effective treatment before it is discharged into the environment. From there, the Atlantic Ocean becomes the final receiver, as coastal outfalls release treated or partially treated effluent directly into marine waters. The molecules disperse, dilute and persist. Small organisms will absorb them first, then fish that consume those organisms, then predators that consume the fish. Sharks sit at the top of this literal chain reaction, integrators of everything below them in their predator role, accumulating what the system carries. Their bodies, thus, become records of exposure.
Sertraline is particularly interesting in this context because of how it behaves chemically. It is lipid soluble, meaning it can bind to fatty tissues, making it more likely to persist in organisms with high fat content (including neural tissue). Finding it in shark brains just confirms exposure, not effect, as we do not fully understand what it is doing there. And yet, the possibility of effect is what makes this new finding difficult to ignore. According to Adjunct Professor Dr. Mariana Batha Alonso of the Instituto de Biofísica Carlos Chagas Filho, Universidade Federal do Rio de Janeiro and UFRJ researcher Leonardo Vazquez, the authors of The Conversation piece, studies in laboratory settings have shown that sertraline can alter behavior in aquatic species, like zebrafish exposed to concentrations comparable to those found in coastal waters that showed changes in movement and learning (specifically developing “hypokinesia and learning delays, along with alterations in the serotonergic system”). But sharks are not zebrafish — their neurobiology is distinct, shaped by an evolutionary lineage far older than most modern contaminants. They are cartilaginous fish with physiological traits that in some ways share surprising similarities with mammals and differ significantly from bony fish, raising to mind a complicated question: if a pharmaceutical can tap into neurological pathways that are shared across vertebrates, what does that imply for a species like a shark? As of now, we do not yet know… which is a problem, because this particular scenario is playing out far beyond Rio, with similar studies in other regions detecting a range of human pharmaceuticals in marine animals, from caffeine to painkillers and even illicit drugs. This is no longer an isolated contamination event but a broader patterns that is reflecting back human chemical usage in ways that were not anticipated when modern wastewater systems were designed.
It should be noted that this is not about blaming medication or the people who rely on it. In Brazil, as in many countries, antidepressant use has risen as access to mental health care improves and more people (rightfully) receive support. But every system has downstream effects, and those effects do not stop at the edge of a drainage pipe. So what do we do with this knowledge that a molecule designed to help human minds may be reaching the brains of apex predators?
We cannot yet say what sertraline is doing inside a shark’s brain. We can say it is there and that this pathway between pharmaceuticals and marine predators exists. We can say our infrastructure was not designed with this outcome in mind. Meaning the next step is acknowledging that the infrastructure gap is now a scientific and policy problem, not just a curious find that will be published in a scientific journal article. For those in power, that means rethinking wastewater systems so they are capable of removing pharmaceutical micropollutants, not just organic waste and pathogens. It also means expanding monitoring programs to routinely track drugs in coastal waters and in sentinel species like sharks, rather than treating these findings as one-off surprises. And, at the same time, policymakers should be calling for better ecotoxicology research to understand what chronic, low-level exposure actually does in real marine ecosystems (not just in laboratory settings); without a baseline, we are essentially flying blind while continuing to release biologically active compounds into the ocean.
While this is largely a systems-level issue tied to wastewater treatment, regulation and industrial design, so what the every day person can do to help is limited, “limited” is not the same as “nothing.” One of the most direct things people can do is avoid flushing or discarding unused medications down the sink or toilet! Many countries have pharmacy take-back programs specifically designed to keep pharmaceuticals out of wastewater streams — check to see if you have something similar near you. Public support for upgrading sewage infrastructure and treating pharmaceuticals as “emerging contaminants” will also help shift what governments prioritize when allocating funding (as wastewater upgrades and pharmaceutical filtration technologies already exist but are not always implemented because they are expensive and not always mandated).
In the end, no one is expected to solve this individually. But the combined efforts of the above moves a problem like this from being documented to being addressed.
