The hammerhead shark is one of the most ancient marine species on the planet, and it is very common along the Brazilian coast. Martin Prochazkacz/ShutterstockSertraline is one of the most widely prescribed antidepressants in the world. Global sertraline sales are expected to keep growing, projected to expand from an estimated US$1.94 billion in 2025 to approximately US$3.13 billion by 2032.In Brazil, a national survey indicates that 10.2% of Brazilians were diagnosed with depression, and it is estimated that 4% used antidepressants.But what few people know is that some of these pills take a second path after being metabolized by the body: They are excreted in urine, enter the sewage system, and go straight into the ocean.And in Rio de Janeiro, in addition to all the beauty of the ocean, there are also sharks.The finding no one expectedThe EcoShark Project, which is coordinated by Mariana Alonso, a professor at the Carlos Chagas Filho Institute of Biophysics at UFRJ, has been monitoring shark health along the Rio de Janeiro coast since 2018. This research also involved other scientists, such as José Souto-Neto and Victor Alves, and is a pioneering initiative on emerging contaminants in elasmobranchs.The study, to be submitted for publication, but already shared among UFRJ scientists, identified sertraline — the active ingredient in Zoloft and dozens of generic versions — in the brain tissue of hammerhead sharks (Sphyrna lewini and S. zygaena), classified as critically endangered species (IUCN, Ibama)The hammerhead sharks tested had been accidentally caught in fishing nets off the coast of Recreio, Barra da Tijuca, and Copacabana beaches and sent to analysis thanks to a partnership between fishermen and researchers from UFRJ. As apex predators, sharks are highly susceptible to bioaccumulation and biomagnification of contaminants in the food chain. As a shark swims and feeds, it constantly takes in amounts of contaminants from the water and sediment, while it also absorbs all the contaminants its preys, like other fish and cephalopods (squids and octopus), took in before as they also swam and fed. And, increasingly, the contaminants in these food chains are the residues of our medications.The path of the medicineHow does a human antidepressant reach a shark’s brain? The path is less surprising than it seems.When a person takes sertraline, the body metabolizes most of the drug in the liver. Sertraline can be excreted unchanged or metabolized, and both forms end up in the sewage system.Conventional wastewater treatment plants were designed primarily to remove organic matter, nutrients, and microorganisms. And the removal of pharmaceutical compounds is often incomplete. As a result, traces of antidepressants and their metabolites are detected in treated effluents and in aquatic environments.In the Brazilian state of Rio de Janeiro, only about 47% of the sewage generated is effectively treated, according to recent data from Brazil’s National Sanitation Information System.A significant portion of the sewage is discharged into the ocean via the Ipanema and Barra da Tijuca submarine outfalls. With only preliminary treatment, these systems fail to remove pharmaceuticals, releasing molecules into the coastal environment, where they are absorbed by fish and marine invertebrates directly from the water or through their food.In sharks, various contaminants accumulate in specific tissues, especially the liver. In the case of sertraline, its affinity for lipid-rich tissues and the nervous system may help explain its detection in the animals’ brains.This is not an isolated caseRio de Janeiro is not alone on this map. In March 2026, a study published in the journal Environmental Pollution revealed that 28 of 85 sharks sampled near Eleuthera Island, in the Bahamas, had detectable concentrations of cocaine, caffeine, and painkillers in their blood.Brazilian researchers tested samples and found painkillers and cocaine in sharks. The finding has shifted perceptions: If drugs are found in sharks off a Caribbean island with low urban density, what can we expect from those swimming less than 1 km (0.6 miles) from Rio’s beaches? The study also detected physiological changes in the animals, suggesting that these substances may affect their biochemistry.Why the shark’s brain is the problemSertraline, which acts on serotonin in the human brain, was detected in shark brain tissue. This points to exposure and bioaccumulation. Since this serotonin transporter is very similar across vertebrates, the drug could theoretically interact with the animals’ proteins. But detection alone does not yet allow us to conclude that there has been a behavioral or physiological change in the animals.Science already shows that sertraline can affect other marine animals: in the laboratory, zebrafish exposed to 0.1 µg/L of sertraline — a concentration found in coastal waters — developed hypokinesia and learning delays, along with alterations in the serotonergic system.What is not yet known — and this is the question EcoShark is trying to answer — is what these compounds do to an elasmobranch. Sharks have neurochemistry distinct from that of bony fish, more similar to that of mammals. The answer, for now, is a question mark weighing hundreds of pounds.A question that cannot be ignoredBrazil has the highest fatality rate from shark incidents in the world. In 2021, the recorded mortality rate reached 30% — compared to 1% in the United States and 14% in Australia.No, we’re not suggesting that antidepressants in the oceans cause attacks. But the question science has a duty to ask is another: If these drugs, at relevant concentrations, alter the behavior of fish in the lab, what actually happens to sharks chronically exposed to them in the world’s most polluted coastal areas?What’s at stake beyond the obviousThe discovery of sertraline in the brains of hammerhead sharks in Rio de Janeiro touches on three crises that Brazil still treats as separate.The first is the mental health crisis. Antidepressant use in Brazil rose by 12.4% among adults aged 29 to 58 between 2023 and 2025. This increase is not a problem in itself. It reflects progress in diagnosis and access to treatment. But each pill has a second destination that is not being monitored.The second is the sanitation crisis. As long as about half of Rio de Janeiro’s sewage remains untreated — without a process capable of removing pharmaceutical compounds — the ocean will continue to serve as a dumping ground for our household medicine cabinets.The third crisis is one of conservation: The hammerhead shark, a critically endangered species, is essential to marine balance — its presence regulates and stabilizes the food chain. Altering this animal’s neurochemistry is an involuntary and uncontrolled experiment.What needs to changeThree actions are urgent and are not mutually exclusive. Brazil’s environmental monitoring protocols must include the systematic tracking of pharmaceuticals in sharks, rays, and cetaceans. The methodology already exists — namely, the EcoShark and EcoDELFIS projects. What’s missing? Ongoing funding and a public policy that values and recognizes pharmaceuticals as emerging pollutants.The country’s wastewater treatment plants need to be modernized to remove pharmaceutical micropollutants.Funding for marine ecotoxicology research needs to be expanded. Brazil has a coastline of nearly 8,000 kilometers (5,000 miles), one of the planet’s most biodiverse regions, and now, sharks with antidepressants in their brains. Sertraline was created to alleviate human suffering. The fact that it has reached the nervous system of a predator just a few kilometers from Copacabana beach is the clearest indication of the extent to which this generation is leaving its mark.The research was funded by PIBIC-UFRJ, Capes, and Faperj. The implementation of the EcoShark Subproject within the Marine and Fisheries Research Project was a compensatory measure established by the Conduct Adjustment Agreement under the responsibility of the company PRIO, led by the Federal Public Prosecutor’s Office – MPF/RJ.Mariana Batha Alonso received funding through FUNBIO for the EcoShark Subproject, part of the Marine and Fisheries Research Project, which is a compensatory measure established under the Conduct Adjustment Agreement for which PRIO is responsible, overseen by the Federal Public Prosecutor’s Office (MPF/RJ).Leonardo Vazquez received funding from FAPERJ/CNPQ.