The oxalate salt of escitalopram, a pure S-enantiomer of the racemic bicyclic phthalane derivative citalopram, with antidepressant activity. As a selective serotonin reuptake inhibitor (SSRI), escitalopram blocks the reuptake of serotonin by neurons in the central nervous system (CNS), thereby potentiating CNS serotonergic activity.