The capability of selected pharmaceuticals to act as nontraditional endocrine disrupting chemicals in the environment has been poorly documented. The widely prescribed antidiabetic metformin has the potential to accumulate and persist in water bodies, with unknown consequences for aquatic biota. This study reports the biological responses of a sentinel organism, common mussel Mytilus edulis, exposed to concentrations of metformin commonly found in the aquatic environment, coupled with heat stress. The results indicate that metformin has the potential to increase the vitellogenin mRNA expression, to inflict severe pathologies of the gonad and to destabilize the lysosomal membrane in the hemocytes. Overall, this article suggests that metformin should be considered an emerging contaminant of concern and further research should focus on its mechanisms of action.