Vitamin B12 or cobalamin (aka hydroxocobalamin, cyanocobalamin, methylcobalamin, and adenosylcobalamin depending on the particular vitamer) is critically important in keeping your body functioning and healthy. It mainly acts as an intermediate, or co-enzyme in reactions that occur in a huge number of the body's metabolic processes.
Relatively mild deficiency of B12 can result in people feeling tired, due to a reduction in Red Blood Cell (RBC) production. RBCs carry oxygen from the lungs to the tissues, and Carbon Dioxide in the reverse direction.
Moderate deficiency can result in a sore tongue, mouth ulcers, breathlessness, hair falling out, and strange sensations like 'pins and needles' .
Severe deficiency can result in irritability, poor memory, reduced heart function, changes in reflexes, depression, and anxiety.
Most people don't need to worry about B12 deficiency, but if you are deficient, it's probalbly caused by an in ability to absorb B12 from the gut. This tends to occur as we get older and this is reflected in the stats around B12 deficiency - according to one study 12% of people older than 65 in New Zealand are B12 deficient.
Some medicines can reduce absorption of B12 the most commonly used medicine in NZ is Metformin.