A method of treating a central nervous system disease, the method including administering an agent including a therapeutically effective amount of vitamin B12 to a patient with a central nervous system disease to treat the central nervous system disease. This disclosure also relates to a method of promoting M2 macrophage/microglia induction, inhibiting M1 macrophage/microglia induction, and/or reducing the ratio of M1 macrophage/microglia to M2 macrophage/microglia in a patient in need thereof, the method including administering an agent including a therapeutically effective amount of vitamin B12 to the patient to promote M2 macrophage/microglia induction, inhibit M1 macrophage/microglia induction, and/or reduce the ratio of M1 macrophage/microglia to M2 macrophage/microglia. This disclosure also relates to a pharmaceutical product including vitamin B12.