The current protocol seeks to expand upon the investigators previous work by opening recruitment on a pilot study of creatine 10g daily vs. placebo as a treatment for female adolescents with SSRI-resistant MDD. The purpose of the pilot study is twofold: A) to evaluate several aspects of the feasibility of creatine supplementation as a treatment for this population; and B) to estimate the effect size of adjunctive creatine, to inform the design and implementation of a potential future efficacy trial.