B12 is a B vitamin that is found only from animal sources and supplements. If your digestion is at all compromised, it can be hard to absorb. It is necessary for absorbing folate from food, and for making methyl groups.
Reasons to Suspect
If you don't have adequate stomach acid or your digestion is otherwise not doing well, if you don't eat meat, if you're vegan, or if you have a blood test that says your B12 is low, a supplement is likely in order.
Meat, fish, milk, eggs. In supplement form, a methyl- or hydroxy- sublingual B12 will be better than the cyano- form, and/or a swallowed pill.