Are we accidentally “drugging” different species of fish?

The impact of human activities on natural ecosystems is now a widely documented reality. Among the most well-known forms of pollution are plastic, pesticides, and industrial discharges, but in recent years, another form of contamination has emerged more clearly—one that is more silent, less visible, and therefore often overlooked: pharmaceutical pollution. Every time we take a medication, such as an antibiotic, an antidepressant, or an anxiolytic, a portion of it is not absorbed by our body and ends up—through urine—in the sewage system. Wastewater treatment plants, although capable of removing many pollutants, are unable to completely filter out these chemical substances, which end up accumulating in rivers, lakes, and other aquatic habitats. The drugs used to treat anxiety, infections, or chronic disorders thus come into contact with animals living in these environments, altering in still poorly understood ways their behavior, physiology, and, in some cases, even their survival.

Among the most common substances found in wastewater are caffeine, metformin (a diabetes drug), the hormones contained in contraceptives, traces of cocaine, various antibiotics, analgesics such as paracetamol, and especially psychiatric medications like antidepressants and benzodiazepines. The concentration of these molecules in the environment is generally very low, to the point that a human accidentally drinking a glass of river water would not suffer any evident effects. But for aquatic organisms, often much smaller and more sensitive, even micro-doses can have significant effects. As reported by Vox, a 2007 study showed that contact with minimal amounts of synthetic estrogens, such as those found in birth control pills, can compromise the reproductive ability of male fish—causing, in the long term, the collapse of individual populations. In other cases, medications affect behavior: some antidepressants increase fish boldness, while others alter their aggression. Even benzodiazepines seem to have an anxiolytic effect not only on humans but also on animals: fish that involuntarily ingest microdoses of drugs like oxazepam or clobazam become more inclined to explore their territory and less prone to forming groups, thus potentially making them more vulnerable to predators.

A recent study conducted in Sweden, published in the journal Science, analyzed the effect of clobazam, a benzodiazepine commonly used to treat anxiety and epilepsy, on a sample of young Atlantic salmon during their migration to the sea. The researchers implanted devices in the fish that released small doses of the drug, comparable to what might be found in contaminated waterways. The salmon were then released into a river that included two dams—a particularly stressful obstacle for these animals. The results showed that the salmon exposed to clobazam were more numerous among those that managed to complete the journey and tended to overcome the obstacles more quickly than their counterparts that had not taken the drug. The researchers hypothesize that this difference is due to reduced sociability and a greater propensity for risk: the "sedated" fish tended to swim alone rather than in groups and to pass through the dams without much hesitation, perhaps also because they were less afraid of predators. As the study's authors explained, any variation in the instinctive behaviors of a species can have repercussions on the entire ecological balance.

The case of clobazam represents only the tip of the iceberg. The pharmaceutical substances released into the environment are thousands, and for most of them, there are no in-depth studies on the effects on ecosystems. Faced with this complexity, several experts highlight the urgency of considering pharmaceutical contamination as a real ecological threat, on par with habitat destruction or climate change. Even if visible effects are still unclear, the very fact that a species' behavior is altered by a chemical substance designed for another purpose should be enough to define it as a form of pollution.