Pharmaceutical pollutants pose a threat to ecosystems worldwide. Yet, few studies have considered the interaction between pharmaceuticals and other chronic stressors contemporaneously, even though the environmental challenges confronting animals in the wild seldom, if ever, occur in isolation. Thermal stress is one such environmental challenge that may modify the threat of pharmaceutical pollutants. Accordingly, we investigated how fluoxetine (Prozac), a common psychotherapeutic and widespread pollutant, interacts with temperature to affect life history traits in the water flea, Daphnia magna. We chronically exposed two genotypes of D. magna to two ecological relevant dosages of fluoxetine (30ng/L and 300ng/L) and a dosage representing levels used in acute toxicity tests (3000ng/L), and quantified the change in phenotypic trajectories at two temperatures (20°C and 25°C). Across multiple life history traits, we found that fluoxetine exposure impacted the fecundity, body size and intrinsic growth rate of D. magna in a non-monotonic manner at 20°C, often in genotypic-specific ways. At 25°C, however, the life history phenotypes of individuals converged under the widely varying levels of fluoxetine, irrespective of genotype. Our study demonstrates the importance of considering a variety of ecological factors when assessing the effects of pharmaceutical pollutants, as elements such as temperature or genotype have the potential to greatly influence results.