You need vitamin B12 for many important processes inside your body, particularly for making red blood cells and keeping your nerves healthy. If you have a short supply of vitamin B12, your body won't make as many red blood cells, they will be abnormally large and won't last for as long as they should. If you don't have enough red blood cells, the tissues and organs of your body may not be getting enough oxygen. This leads to the symptoms of anaemia.
Attribution: BUPA