Vitamin B12 is vital for red blood cells to divide and become active. These red blood cells carry oxygen around the body. A lack of vitamin B12 can lead to fatigue due to the lack of oxygen-carrying capacity in the blood. This is called megaloblastic anemia.
Apart from fatigue, other symptoms of B12 deficiency can include rapid breathing or shortness of breath, headaches, indigestion, loss of appetite, palpitations, problems with your vision, feeling weak, diarrhea, a sore or red tongue, problems with memory, understanding and judgment (cognitive changes).