Vitamin B12 is essential for the division and activation of red blood cells. These red blood cells carry oxygen around the body. A lack of vitamin B12 can cause fatigue due to a lack of oxygen-carrying capacity in the blood. This is called megaloblastic anemia.
In addition to fatigue, other symptoms of B12 deficiency include rapid breathing or shortness of breath, headache, indigestion, loss of appetite, heart palpitations, problems with your vision, weakness, diarrhea, sore or red tongue, memory loss, and problems with perception and judgment (cognitive changes).