Contamination of the environment by pharmaceutical pollutants poses an increasingly critical threat to aquatic ecosystems around the world. This is particularly true of psychoactive compounds, such as antidepressant drugs, which have become ubiquitous contaminants and have been demonstrated to modify aquatic animal behaviours at very low concentrations (i.e. ng/L). Despite raising risks to the hydrosphere, there is a notable paucity of data on the long term, multigenerational effects of antidepressants at environmentally realistic concentrations. Moreover, current research has predominantly focused on mean-level effects, with little research on variation among and within individuals when considering key behavioural traits. In this work, we used a multigenerational exposure of a freshwater snail (Physa acuta) to an environmentally relevant concentration of the antidepressant fluoxetine (mean measured concentration: 32.7 ng/L, SE: 2.3). The snails were allowed to breed freely in large mesocosm populations over 3 years. Upon completion of the exposure, we repeatedly measured the locomotory activity (624 measures total), reproductive output (234 measures total) as well as morphometric endpoints (78 measures total). While we found no mean-level differences between treatments in locomotory activities, we did find that fluoxetine exposed snails (n = 46) had significantly reduced behavioural plasticity (i.e. VW; within-individual variation) in activity levels compared to unexposed snails (n = 32). As a result, fluoxetine exposed snails demonstrated significant behavioural repeatability, which was not the case for unexposed snails. Further, we report a reduction in egg mass production in fluoxetine exposed snails, and a marginally non-significant difference in morphology between treatment groups. These results highlight the potential detrimental effects of long-term fluoxetine exposure on non-target organisms at environmentally realistic dosages. Additionally, our findings demonstrate the underappreciated potential for psychoactive contaminants to have impacts beyond mean-level effects, with consequences for population resilience to current and future environmental challenges.