An object of the present invention is to provide a drug useful in the treatment of nervous system diseases.The drug containing vitamin B12 as an active ingredient according to the present invention has an action of promoting M2 macrophage/microglia induction, an action of suppressing M1 macrophage/microglia induction, an action of promoting nerve regeneration and the likes, and It is very useful as a therapeutic agent for nervous system diseases, particularly central nervous system diseases such as cerebral infarction, dementia and spinal cord injury.本發明之課題係提供一種於治療神經系統疾患有用之藥劑。本發明之以維生素B12作為有效成分之藥劑具有促進M2巨噬細胞/小神經膠質細胞誘導作用、抑制M1巨噬細胞/小神經膠質細胞誘導作用、神經再生促進作用等，並且該藥劑作為神經系統疾患，尤其是作為腦梗塞、失智症、脊髓損傷等中樞神經系統疾患之治療劑非常有用。
