 [ Symptoms ] i feel worn out can not sleep nauseous and cold [ Disease ] it depends on why you are taking it if it is for b12 deficiency then you take vitamin b12 but with a doctor is guidance to monitor blood levels if you are taking it for high blood levels of homocysteine or to prevent age related macular degeneration then you take a combination of b12 folic acid and pyridoxine for more information on dosing side effects and interactions see this link