Vitamin B12 plays an essential role in red blood cell formation, nerve function, cell metabolism and DNA production. It can be found in meat, poultry, fish and dairy, or as a supplement. Your body can store vitamin B12 for many years so most individuals will not have a B12 deficiency. However, the following people may be more prone to deficiency: vegetarians/vegans, older adults, or people with digestive tract conditions that affect the absorption of nutrients. Vitamin B12 deficiency can lead to fatigue, anemia, muscle weakness, intestinal problems, nerve damage, and mood disturbances. Visit your physician or pharmacist to discuss whether you should be taking a vitamin B12 supplement.