While most do not, some people do need to take a B12 supplement. Talk to your doctor or pharmacist to see if you are at risk of deficiency and to determine the best supplementation regimen.
Dietary sources of B12 include animal-based products (meat, dairy, and eggs) and fortified cereals. As such, vegans and strict vegetarians are at particular risk of deficiency. Deficiency may lead to anemia and rarely to neuro-psychiatric problems.
Another cause of deficiency is decreased ability to absorb dietary B12, which may result from lowered stomach acidity (age > 60 years, long term use of stomach acid-lowering medications, or gastric surgery); problems, inflammation, or infection involving the pancreas or the intestinal tract; and also treatment with metformin. These conditions are generally easily treated by oral B12 tablets.
Occasionally, more complicated auto-immune problems exist and require treatment using a sublingual or injectable form of B12.
Talk to your doctor or pharmacist to see if you need to be worked up for possible B12 deficiency and require supplementation.