Wernicke's encephalopathy was first described by Wernicke in 1881. However, the disease is often overlooked in our daily clinical activities, since Wernicke's encephalopathy can not be definitely diagnosed in an emergency room. If Wernicke's encephalopathy is not treated properly with vitamin B_1, patients will usually suffer from Korsakoff syndrome or sometimes fall into death. Vitamin B_1 deficiency has been thought to be overcome by an abundant food supply or effective vitamin B_1 treatment. Increased alcohol consumption or unbalanced diet has introduced a new aspect of vitamin B_1 deficiency, especially Wernicke's encephalopathy which often leads to dementia. To prevent dementia due to Wernicke's encephalopathy, it has been recommended that a large dose of vitamin B_1 is administered to patients with consciousness disturbance. This article summarizes the pathophysiology, diagnosis and symptoms of Wernicke's encephalopathy as well as the dose of vitamin B_1 administered to patients with Wernicke's encephalopathy and the side effect of the vitamin B_1 therapy.