America's Compounding Center says the following about B12:
"Cobalamin, also known as Vitamin B12 or cyanocobalamin, is a complex B vitamin involved in DNA synthesis, and in the conversion of food into energy. Dietary sources of cobalamin include meat, poultry, clams, oysters, liver, and dairy. Since cobalamin is stored in the body, deficiency is rare and takes about three years for the body to develop. Cobalamin deficiency most commonly develops in patients with chronic malabsorption syndromes, genetic disorders, and vegetarians who do not consume any animal products. Complications of cobalamin deficiency include anemia (low red blood cell count), fatigue, muscle weakness, poor coordination, confusion, and tingling or numbness in the hands and feet. Cobalamin is available in oral, injectable, or intranasal dosage forms. A common oral dose for patients with cobalamin deficiency ranges from 100-1,000 micrograms (mcg) daily. Potential side effects of cobalamin supplementation include itching, diarrhea, headache, and anxiety."