Vitamin B12 is needed to make new cells in the body such as red blood cells. A lack of vitamin B12 leads to anaemia, low energy, stomach problems (constipation, loss of appetite, weight loss), poor memory and nerve damage.
Most people get enough vitamin B12 from their diet. It is is found in meat, fish, eggs, and milk. It is generally not found in plant foods but many foods have added vitamin B12 such as breakfast cereals. A normal balanced diet usually contains enough vitamin B12.