Muscle weakness is one of the common symptoms of vitamin B12 deficiency. Vitamin B12 is a vital ingredient in the manufacturing of red blood cells, which are responsible for the transportation of oxygen to all muscles in the body. Once in the muscles, oxygen is used in the production of energy in the form of ATP. Therefore, the lack of vitamin B12 may cause fatigue since the muscles are not receiving enough oxygen for energy production. The muscles, when they do move, often feel sluggish to the individual with a vitamin B12 deficiency. Someone who could previously carry heavy things will likely find their abilities significantly diminished.