Inclusion Criteria:

• Adults age 18-65 years inclusive
• Current diagnosis of MDD identified by the MINI (Mini Neuropsychiatric Interview)
• Current HAM-D17 score of >= 16
• Adequate adherence to any FDA approved SSRI or SNRI for at least 8 weeks
• Right-handed
• Residing at > 4000 ft for at least 12 weeks

Exclusion Criteria:

• Any non-MDD and non-anxiety psychiatric diagnosis, as identified by the MINI
• History of or current diagnosis of renal disease, such as chronic renal failure, acute renal failure or end stage renal disease
• Current colitis or diverticulitis
• History of or current pulmonary disease (except well controlled asthma)
• Current smoking
• History of cardiac disease or QTc > 500ms
• History of fibromyalgia or any rheumatological condition
• History of or current seizure disorder
• Current serious suicide risk identified by the Columbia Severity Suicide Rating Scale
• Current treatment with an antipsychotic, mood stabilizer, or non-SSRI/SNRI antidepressant except for bupropion at FDA-approved doses or trazodone up to 200mg, or use of any supplements apart from standard multivitamins
• Positive pregnancy test, pregnancy, failure to use adequate birth control method
• Previous diagnosis of serotonin syndrome or evidence of serotonin syndrome
• Pre-existing eosinophilia (absolute eosinophil count > 500/uL)
• Contraindications to MRI: ferromagnetic implants, implanted devices, claustrophobia