Adults should take 2.4 micrograms of vitamin B12 each day. Adults more than 50 years old should eat foods fortified with B12 or take a 25- to 100-microgram vitamin B12 supplement daily. Pregnant women should take 2.6 micrograms daily, recommends the Mayo Clinic.
Vitamin B12 is essential to DNA synthesis and is found in meat, eggs, fish, shellfish and dairy products. It is rare for vitamin B12 levels to be low in healthy adults, since the body can store it for several years in the liver. Vegetarians, elderly people, and people with HIV are more likely to have decreased vitamin B12 levels, according to the Mayo Clinic.