The present invention relates to an isotopologue of Compound 1 substituted with deuterium at the methylene carbon of the benzodioxol ring. The isotopologues of this invention selective serotonin reuptake inhibitors (SSRIs) and possess unique biopharmaceutical and metabolic properties compared to Compound 1. They may also be used to accurately determine the concentration of Compound 1 in biological fluids and to determine metabolic patterns of Compound 1 and its isotopologues. The invention further provides compositions comprising these deuterated isotopologues and methods of treating diseases and conditions that are responsive to increased neuronal serotonin transmission, alone and in combination with additional agents.