Vitamin B12 plays a major role in a number of the body’s essential functions, including DNA synthesis and the formation of red blood cells. It also helps the body convert fats and proteins into energy and aids in the breakdown of carbohydrates. 

B12 deficiency can lead to several ailments, most notably megaloblastic anemia, which is caused by a low red blood cell count. The most common symptom of megaloblastic anemia is fatigue. This form of anemia, as well as other health issues associated with B12 deficiency, can be easily treated with injections of the vitamin. This fatigue occurs because your body doesn’t have enough vitamin B12 to make red blood cells, which transport oxygen throughout your body. As a result, you are unable to efficiently transport oxygen to your body’s cells, making you feel tired and weak.