B12 is not like other vitamins. It is a large molecule with many opportunities for malabsorption or failure to use properly. Possibly the most striking difference between B12 and other vitamins: normally it is stored in quantities necessary for years of use; thus the symptoms of deficiency can come on very subtly and slowly for a long time before becoming alarming.
By the time the patient becomes alarmed, symptoms may be a strange collection of seemingly unrelated problems. Because this is not seen by many doctors as feasible, the patient may be patted on the head and told to go home and relax. In my case, and that of others with severe malabsorption, taking that kind of advice is deadly.
Too much B12 is not a problem. Too little is a big problem, and long-untreated deficiency is damaging to deadly. Most medical people have no idea how many different ways damage can vary from patient to patient. Only a partial list of possible symptoms and signs is possible, and that list is very long.
Unfortunately most medical articles mention only a few of the possible symptoms, and too many doctors infer that those are "the" symptoms.
Also, because many papers concentrate on edlerly patients, people often assume that it is only a problem for the elderly. Middle-aged and young people can be damaged severely by B12 malabsorption, and some babies are born with defects preventing normal use of B12.
This site provides an overview that is better than most, and in language that is quite understandable:
This is a more detailed article, from the American Family Physician:
More to come...