When your body is unable to absorb vitamin B12 from your diet due to the condition known as pernicious anaemia, you may experience vitamin B12 insufficiency. It may also occur if your diet is deficient in foods high in vitamin B12 or if you are using drugs that prevent the absorption of this vitamin. Symptoms of vitamin B12 deficiancy include; extreme tiredness, lack of energy, muscle weakness, disturbed vision, low mood, hair loss and premature greying of hair.