Pharmaceuticals from human wastewater continue to pollute oceans as treatment technologies fail to filter out many of these contaminants and their byproducts. Selective serotonin reuptake inhibitors (SSRIs) are a class of antidepressant pharmaceuticals that, due to their increasing prescription rate, are emerging as a pollutant of concern in marine waters. Research has suggested exposure to some SSRIs can be harmful to marine species, but this has not yet been tested at environmentally relevant concentrations. Our study investigated the potential impact of citalopram, a SSRI in marine waters with limited information about its biological effects. We used a model invertebrate species (amphipod Allorchestes compressa) to determine the lethal concentration (LC50) of citalopram and explored how it influences behaviour using sublethal endpoints (movement, phototactic responses, and respiration). We found the LC50 of citalopram to be higher than what is detected in Port Phillip Bay coastal waters. However, preliminary results indicate changes in behavioural endpoints, suggesting that exposure to citalopram may alter amphipods’ activity. This study highlights how citalopram disrupts physiological and behavioural processes which may have implications for populations. We explore how these findings can be used to inform environmental guidelines and reduce the impact of pharmaceutical pollution in the marine environment.