Apparently, no one knows very much about B-12. Pretty much, if they find out you're low on B-12, they will inject you with the stuff once a month for the rest of your life. Not a very pleasant future to look forward to.
Since I can't handle injections very well (I do okay with blood being taken out, but I pass out just about every time I get injected) I decided to do a little more research.
I found one tiny little study (36 people) which seemed to indicate that sublingual and oral pills of 500+ micrograms taken daily were as efficient as an injection when taken regularly. A few of their patients may have had missing intrinsic factor.
What causes B-12 deficiency? There are a lot of theories, but most people with severe deficiencies probably have multiple causes or a lack of intrinsic factor. Intrinsic factor is just this weird thing in our stomachs that helps us absorb the B-12 while the food is in our small intestine. Pretty weird. It also might help us to be able to reabsorb any extra B-12 in our bodies not from food, although that is debated among scientists.
I personally believe that it's possible for us to "use up" more B-12 than usual during extremely stressful periods of life, although no scientists or doctors really have much information about that. The main suspected cause of B-12 deficiency in people with normal intrinsic factor is other stomach problems like Crohn's disease or celiac disease. Certain medications might interfere with B-12 absorption as well.
Bottom line - if you are experiencing any unusual symptoms - including memory loss, depression, fatigue, numbness in hands and feet: see your doctor and be sure that she runs a B-12 test (it takes two blood tests) in addition to looking at other potential causes.