- Pharmaceuticals have a wide range of detrimental side effects on people. Scientists also know that pharmaceutical pollution is widespread in aquatic ecosystems, largely due to wastewater outflows and runoff.
- Studies now show pharmaceutical waterway contaminants can accumulate in aquatic insects at various life-cycle stages. These pollutants can then be transferred to terrestrial ecosystems as the insects are consumed by other species, including birds and bats.
- Research also shows that pharmaceuticals can cause changes in the physiology and behavior of insects, with potential knock-on effects for populations and wider ecosystems.
- But the full consequences of the transfer of a wide range of pharmaceutical contaminants to aquatic insects, and then via their predators to terrestrial environments and food webs, is largely unknown.
Pharmaceuticals that people globally rely on for daily health — including antibiotics, antidepressants and painkillers — are entering ecosystems via wastewater, posing poorly understood risks. Once there, aquatic insects can accumulate these chemical contaminants, with their predators transporting them back onto land, raising concern among scientists.
A recent paper found that a host of pharmaceuticals can accumulate in winged species such as caddisflies and mayflies at different life stages, with the quantity varying among species.
The paper’s first author, Marek Let, from the University of South Bohemia, says pharmaceutical pollution might not be as concerning as pesticide contaminants because medical drugs are generally found in low concentrations.
But he adds that some pharmaceutical compounds can be incredibly toxic.
These include sertraline (the active ingredient of the commonly prescribed antidepressant Zoloft), which his study found can bioamplify in caddisflies, increasing in concentration as the insect passes through its life cycle. Other antidepressants, including norsertraline and venlafaxine (the active ingredient in the popular prescription drug Effexor), as well as theophylline (prescribed to treat asthma symptoms) also bioamplify in caddisflies.
“This study also proves that the pharmaceuticals do not stay in the water, and they can be emitted into the terrestrial environment and contaminate some terrestrial predators, like birds or bats,” Let says.
Michael Bertram, a behavioral ecologist and ecotoxicologist at the Swedish University of Agricultural Sciences, who wasn’t involved in the new research, says the recent study is important because it shows how aquatic insects are exposed to pharmaceuticals at different life-cycle stages and how, ultimately, they (and the contaminants they carry) can enter terrestrial food webs.
“This is an important advance because it strengthens the case that pharmaceuticals in wastewater can move from rivers into terrestrial food webs via insect emergence, potentially exposing riparian predators (e.g. spiders, birds, bats) to bioactive compounds,” he wrote in an email.
Tracing the pollution web
The majority of human waste enters into the global environment untreated. Even in areas where sewage treatment exists, facilities don’t completely remove all substances and chemicals, including pharmaceuticals. Studies have found pharmaceuticals in rivers worldwide, even in remote locations in the Brazilian Amazon.
“Our input is so high that we have created pseudo-constant environmental pollutants,” says Ana Previšić, a researcher at the University of Zagreb. Researchers are increasingly working to understand how this pharmaceutical pollution affects insects, along with species that consume them. But that’s a task beset with challenges.
Previšić says it’s not like an algal bloom, for instance, that can leave dead fish in its wake. “Here you have a continuous input in small doses,” she says. This ongoing exposure can cause small, sublethal effects or changes in insect behavior that are difficult to parse out from other changes in the environment. “It’s harder to really say what’s going on.”
In a recent review paper, Previšić’s team found that pharmaceuticals can have a range of detrimental effects on insects — disrupting development, changing behavior and affecting insect microbiomes.
But studies to date also show that some pharmaceutical compounds accumulate more than others and to a differing degree in certain species. Similarly, findings suggest that precisely how any single compound impacts insect species can vary widely.
Further complicating research: While laboratory studies can reveal the effects of individual or groups of pharmaceuticals on insects, lab testing isn’t equivalent to real-world exposure. In rivers and streams, insects are exposed to a watery soup containing numerous pollutants, in varying amounts at the same time, including heavy metals, pesticides, industrial solvents and a plethora of other contaminants.
This makes it very hard to generalize how compounds might impact insects, Previšić says.
But Aneesh Bose, a behavioral ecologist, and Erin McCallum, an ecotoxicologist at the Swedish University of Agricultural Sciences, neither of whom were involved in the current research, say the handful of studies conducted to date suggest that subtle changes caused by pharmaceutical pollutants can ultimately affect both insect communities and wider ecosystem dynamics.
Antidepressants, for example, can cause a wide range of behavior changes in insects. One effect that seems to “pop out again and again” is a reduction in their threat response, Bose says. That means some species may become more adventurous while others are more relaxed and move less — behaviors that could make them more vulnerable to predation.
“It seems to be linked to some inability to evaluate threats in their environment,” he says.
Research by Previšić and her team found that larval Drusus croaticus — a caddisfly species — moved more when exposed to a cocktail of pharmaceuticals and warmer temperatures, causing them to lose body weight. Likewise, similar exposure caused another caddisfly — Stenophylax nycterobius — to lose fat content, particularly in emerging adults.
“That may sound OK that they had a bit less fat,” she says. “But it’s very important for their adult life and reproduction, as their survival depends on the lipids which they have accumulated as larvae.”
These impacts are unlikely to cause a sharp, sudden collapse of an ecosystem, but the effects could cause subtle shifts that ultimately change ecosystem balance. Stenophylax nycterobius, for example, is a species that’s usually present in high numbers in intermittent streams and rivers. Their loss of fat could ultimately reverberate up the food chain, affecting predators including amphibians and birds.
“With every study we see some interesting patterns that might have an ecologically wider meaning,” Previšić says. “It’s very hard to say that, if this is really bad, how bad is it?
Much more research is needed to investigate the possible impacts of pharmaceutical pollutants, she adds, but it’s also important to curb the flow of drugs into the environment.
Unknown threats
The transmission of these contaminants via insects from aquatic environments to terrestrial ecosystems, while rife with uncertainty, still poses potential risk for insect-consuming species, researchers say.
Bertram points to a study from Australia that found 69 pharmaceutical compounds in more than 190 aquatic invertebrate species in streams near Melbourne. The authors estimated that, in theory, species such as platypus (Ornithorhynchus anatinus), which regularly eat these contaminated insects, could be consuming up to half the recommended dose of antidepressants for humans. Other studies have highlighted that waterbirds, such as gulls and terns, are exposed to a broad range of pharmaceuticals, partly due to their diet.
Researchers have only scratched the surface of how pharmaceutical pollution affects aquatic insects and the follow-on effects for species that consume them.
“I don’t think anybody can say with any great certainty what the impacts are on a population level yet,” McCallum says. “And I’d say we’re still very much in the early days of understanding this whole concept of aquatic to terrestrial transfer.”
How this contamination might combine with, or exacerbate, other detrimental human-caused changes in ecosystems — such as climate change and land use change — adds still another layer of complexity for scientists to untangle. That, Bertram says, underlines the need for further research.
“Such work is urgently needed, as failing to account for these interactions risks severely underestimating ecological impacts and misinforming [policymakers and regulators doing] environmental risk assessment and management,” he says.
Banner image: A caddisfly in the Oligotricha genus. Though scientist Marek Let notes that pesticide pollution impacts in waterways are more concerning, he adds that his recent studies show that some potentially toxic pharmaceuticals are accumulating in insects. Image courtesy of Marek Let.
Humans are dosing Earth’s waterways with medicines. It isn’t healthy.
Citations:
Let, M., Grabicová, K., Balzani, P., Musil, M., Roje, S., & Bláha, M. (2025). Bioaccumulation of pharmaceutically active compounds from treated urban wastewaters in aquatic insect larvae and aerial adults. Environmental Science & Technology, 59(10), 5293-5305. doi:10.1021/acs.est.4c13781
Renault, D., Previšić, A., & Derocles, S. A. (2025). Effects of pharmaceuticals and personal care products on insects. Annual Review of Entomology. doi:10.1146/annurev-ento-121423-013316
Bose, A. P., McCallum, E. S., Avramović, M., Bertram, M. G., Blom, E., Cerveny, D., … Brodin, T. (2022). Pharmaceutical pollution disrupts the behavior and predator-prey interactions of two widespread aquatic insects. iScience, 25(12), 105672. doi:10.1016/j.isci.2022.105672
Ložek, F., Kokotović, I., Smolić, A., Bočkor, L., Rožman, M., & Previšić, A. (2025). Integrative effects of warming and contaminants on the physiology and behaviour of coldwater stenotherm caddisflies. Environmental Toxicology and Pharmacology, 119, 104818. doi:10.1016/j.etap.2025.104818
Renault, D., Previšić, A., & Derocles, S. A. (2026). Effects of pharmaceuticals and personal care products on insects. Annual Review of Entomology, 71(1), 635-653. doi:10.1146/annurev-ento-121423-013316
Richmond, E. K., Rosi, E. J., Walters, D. M., Fick, J., Hamilton, S. K., Brodin, T., … Grace, M. R. (2018). A diverse suite of pharmaceuticals contaminates stream and riparian food webs. Nature Communications, 9(1). doi:10.1038/s41467-018-06822-w
Distefano, G. G., Zangrando, R., Basso, M., Panzarin, L., Gambaro, A., Volpi Ghirardini, A., & Picone, M. (2022). Assessing the exposure to human and veterinary pharmaceuticals in waterbirds: The use of feathers for monitoring antidepressants and nonsteroidal anti-inflammatory drugs. Science of The Total Environment, 821, 153473. doi:10.1016/j.scitotenv.2022.153473
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