Hi I've been feeling exhausted losing my hair by the handful and a brain fog for the last few months. My docs did some tests last week and they are saying I'm a little low in vit b12. They have left me a perscription for some tablets, how long does it take for them to start to work ? as I'm just so tired all the time I could literally sleep the clock round I slept from 11pm last night to 10am this morning and I know I could go back to bed and probably sleep most of the day as well.