CASE PRESENTATION: A 54-year-old woman with history of esophagectomy and malnutrition was admitted with vomiting and epigastric pain. Vital signs were normal. Labs were remarkable for a lactate level of 9.8 mmol/L. She was treated with IV fluids with improvement of symptoms, and a decrease in lactate to 3.0 mmol/L. Antibiotics were administered for possible intra-abdominal infection, but CT scan of the abdomen and pelvis did not reveal a source. On hospital day 3, the patient was admitted to the ICU due to hypotension with repeat labs showing pH 7.13 and lactate of 8.7 mmol/L. Neurologic examination revealed blurry vision, confusion, and inability to abduct either eye accompanied by bilateral nystagmus. Wernicke encephalopathy was diagnosed clinically, with acute lactic acidosis due to severe thiamine deficiency. Non-contrasted MRI of the brain showed increased T2 and FLAIR signal along the medial thalami and periaqueductal gray. The patient was treated with 500mg IV thiamine every 8 hours for 5 days, then tapered to complete a 30-day course of 500mg orally. Within 24 hours of initial treatment, her ophthalmoplegia and lactic acidosis resolved. Thiamine level resulted five days later at 15 nmol/L (normal 70-180 nmol/L).