The purpose of this study is to determine whether creatine will be helpful as an adjunctive treatment for treatment-resistant major depressive disorder (MDD) in female and male Veterans. We hypothesize that Veterans receiving creatine will show decreased depressive symptoms as measured by the Montgomery-Asberg Depression Rating Scale (MADRS). We will also use 31-Phosphorus Magnetic Resonance Spectroscopy (31-P MRS) brain scans to compare levels of neurochemicals related to energy metabolism in the brain, before-and-after treatment with creatine, and between healthy controls and MDD participants.