Vitamin B12

The only reliable vegan sources of B12 are foods fortified with B12 (including some plant milks, some soy products and some breakfast cereals) and B12 supplements. Very low B12 intakes can cause anaemia and nervous system damage. Vitamin B12 supplement contributes to the reduction of tiredness & fatigue.

To get the full benefit of a vegan diet, vegans should do one of the following:
Eat fortified foods two or three times a day to get at least three micrograms (mcg or µg) of B12 a day:  

or Take one B12 supplement daily providing at least 10 micrograms 
or Take a weekly B12 supplement providing at least 2000 micrograms 

What Every Vegan Should Know About Vitamin B12  

The Vegan Society