Global production of pharmacologically active compounds exceeds 100 000 tons annually, a proportion of which enters aquatic environments through patient use, improper medicine disposal, and production. These compounds are designed to have mode-of-action (MoA) effects, on specific biological pathways, with potential to impact nontarget species. Here, we used MoA and trait-based approaches to quantify uptake and biological effects of fluoxetine, a selective serotonin reuptake inhibitor, in filter and deposit feeding marine worms (Hediste diversicolor). Worms exposed to 10 mu g L-1, accumulated fluoxetine with a body burden over 270 times greater than exposure concentrations, resulting in similar to 10% increased coelomic fluid serotonin, a pharmacological effect. Observed effects included weight loss (up to 2% at 500 mu g L-1), decreased feeding rate (68% at 500 pig L-1), and altered metabolism (oxygen consumption, ammonia excretion, and O/N from 10 mu g L-1). Bioconcentration of fluoxetine was dependent on route of uptake, with filter feeding worms experiencing up to 130 times greater body burden ratios and increased magnitudes of effects than deposit feeders, a trait-based sensitivity likely as a consequence of fluoxetine partitioning to sediment. This study highlights how novel approaches such as MoA and trait-based methods can supplement environmental risk assessments of pharmaceuticals.