This is an open-label clinical trial of the investigational drug creatine for augmentation treatment of female and male Veterans, ages 18-55, with Major Depressive Disorder (MDD) who have failed to respond to antidepressant treatment with a selective serotonin reuptake inhibitor (SSRI) drug. Based on converging preclinical and animal model research, and our laboratory's prior clinical trials, we hypothesize that the nutritional supplement creatine may provide benefit as an adjunctive treatment to SSRI pharmacotherapy, for Veterans with treatment-resistant depression.