A 73-year-old woman was admitted for scheduled surgery for gastric adenocarcinoma after neoadjuvant chemotherapy, undergoing subtotal gastrectomy with reconstruction by Roux-en-Y gastrojejunostomy. As a previous history there was a probable allergy to vitamin B (Neuromade®). Postoperatively, the patient developed a fistula of the gastrojejunal anastomosis, which was managed conservatively, with cutaneous exteriorisation, forcing the initiation of parenteral nutrition (TPN) without thiamine supplementation. One month later, she presented with neurological deterioration consisting of disorientation, bradypsychia, sleep tendency, horizontal nystagmus and ophthalmoparesis, which led to her admission to the intensive care unit for an intradermal thiamine test. This was negative and he was given controlled administration of thiamine with complete recovery of the clinical picture.