I just read an interesting article on Medscape News that said that Metformin, a medication used to manage Type 2 Diabetes, can reduce the absorption of B12. Higher doses of the medication were more likely to lower B12 levels and people who took a multivitamin were less likely to have low B12. In one of the studies B12 deficiency was reported for 22% of the Metformin users.
B12 is a vitamin that is found in all animal source foods. The body produces a protein called "intrinsic factor" that is necessary for the absorption of B12. Vegans (people who don't eat any animal foods) are at risk for B12 deficiency as are the elderly (who tend to make less intrinsic factor). There are a number of medications and stomach/intestinal problems that can also contribute to B12 deficiency.
Many people are aware that iron is related to anemia, but there are also forms of anemia that are related to B vitamin deficiency. Deficiency of B12 or Folic Acid can both cause anemia as well. Since the symptoms are similar for both B-vitamin caused anemias, a blood test is required to choose the correct treatment. The Merck Manual has more information about B12 and Folate.