Vitamin B-12 deficiency is a blood and neurological disorder caused by a lack of Vitamin B-12 in the blood. The vitamin is required to create red blood cells and to regulate nerve function. As a result, patients with a deficiency of the vitamin start suffering from the symptoms of anemia, and start suffering neurological problems. If the problem is untreated, the neurological damage can be permanent. There are several causes for the deficiency: Tapeworm - In rare cases, B-12 can be absorbed by tapeworms, leaving none for the use of the patient.