Encephalopathy may develop within 1 to 3 weeks upon cessation of thiamine supply. Deficiency of folic acid may require months until encephalopathy develops, whereas this may take years for vitamin B12 deficiency. It may be harmful for the patient if the impairment of cognitive functions is considered to be due to Alzheimer's disease, even though vitamin deficiency is the cause. Differential diagnosis, however, is not easy. Risk factors for vitamin deficiencies include reduction of body weight, associated surgical procedures and nausea during pregnancy.