The increasing use of pharmaceutical drugs in medicine, agriculture, and domestic animals could increase active pharmaceutical ingredients (APIs) in the environment. The ccumulation of APIs in the environment could present an insidious threat to biodiversity, as APIs and their metabolites impact food webs. We assessed the potential effects of a widely used API, orlistat, on aquatic invertebrates. Orlistat, an active ingredient in weight loss drugs, interferes with the digestive enzymes that help breakdown lipids. We reasoned that orlistat could interfere with the assimilation of lipids in aquatic organisms and affect growth, reproduction, and survival. We selected Daphnia magna, a primary consumer, as a model organism for aquatic invertebrates. The experimental treatment involved orlistat dissolved in ethanol added to natural spring water; the control treatments (12.0 µl EtOH/L H₂O) were the same but without the orlistat. In replicated experiments, the survival of Daphnia in orlistat ( ~2 µg/ml) was less than that of controls (p ≤ 0.001). There was also an effect on productivity. We observed delayed egg development, failed egg development, and a lower frequency of neonate births in the orlistat treatment. This suggests that females in the orlistat treatment allocated less of their endogenous resources to reproduction. We also noted subtle differences in growth. The rate of body molts appeared to slow down in the orlistat treatment. We also noted a dose-dependent effect on survival when we increased the concentration of orlistat from 2.38 to 6.95 µg/ml. Although orlistat is hydrophobic, we suspect that it could accumulate in the surface layer, the edges of the littoral zone, and in sediments. We conclude that orlistat can affect aquatic invertebrates. Further research is needed to determine the potential impacts on aquatic food webs and the potential threats to biodiversity.