A 19-year-old Taiwanese man was admitted for psychiatric inpatient treatment because of major depressive disorder. The patient had presented the characteristic symptoms of depressed mood, loss of interest, anorexia, insomnia, and feelings of worthlessness in the preceding 2 months. He had been healthy in the past, without systemic disease such as metabolic disease. Initial laboratory studies revealed random blood glucose, 73 mg/dl (reference range: 70–105 mg/dl), and the thyroid function examination results were all within the normal range. He weighed 80.5 kg and scored 28 on the 17-item Hamilton Depression Rating Scale (Ham-D) on the first hospital day. Initially, he was treated with paroxetine, 20 mg/day, and zolpidem, 10 mg/night, for 14 days. His symptoms of depressed mood, anorexia, and marked insomnia were little improved. Also, he lost 4 kgs in 14 days. After this, treatment with paroxetine was discontinued, and mirtazapine, 30 mg, was prescribed at night. The patient's mood improved, but he also experienced intermittent headaches, increased appetite, and weight gain—6.5 kgs in 1 week. His Ham-D total score improved to 7 in that same week of mirtazapine therapy.