A 30-year-old female with a history of anxiety, cannabis use, and Avoidant/Restrictive Food Intake Disorder presented for residential treatment of a Cannabis Use Disorder. Upon arrival, she had not eaten for two days and was found to be hypotensive with electrolyte disturbances. She was admitted to a nearby hospital, where the Internist diagnosed her with Addison's disease. She was treated with corticosteroid therapy, with rapid normalization of her electrolytes, eating, and anxiety. This is the first published case of undiagnosed Addison's disease presenting as an eating disorder, with cannabis use likely contributing to symptoms. This case elucidates the importance of ruling out other biologic and psychologic causes of clinical presentations before an eating disorder diagnosis can be made.