A 27-year-old woman developed a first episode of psychotic symptoms such as apathy, auditory, visual hallucinations, and persecutory delusions for 5 days. She had a previous personal history of alcohol dependence for the past 2 years. Her weight was 45 kg., with a body mass index of 15. Routine laboratory tests revealed negative findings, as was blood alcohol level. On admission, an initial diagnosis of acute psychotic disorder was made, and she was treated with olanzapine 10 mg nightly. On Day 7, she had diplopia and ataxic gait, whole-body trembling, choreic dyskinesia over perioral muscles, and numbness sensations over four extremities. A thorough neurological examination revealed decreased deep tendon reflexes and muscle strength symmetrically in all four limbs. Meanwhile, nerve conduction velocity showed sensorimotor peripheral polyneuropathy. Computed tomography (CT) of brain revealed a negative finding. More detailed laboratory analysis, including cobalamin and folic acid level, and thyroid function, all were within normal limits. WE was considered in the differential diagnosis, so the olanzapine was discontinued, and oral thiamine 300 mg daily was given on Day 10. On Day 12, auditory and visual hallucinations and persecutory delusions subsided. On Day 14, her diplopia and dyskinesia resolved. On Day 20, her motor coordination, numbness, and muscle weakness also improved. On Day 30, she was discharged and treated with thiamine 300 mg daily without relevant neuropsychiatric alterations for 1-month follow-up.