Vitamin B12 deficiency, also known as hypocobalaminemia, refers to low blood levels of vitamin B12. A wide variety of signs and symptoms may occur including a decreased ability to think and changes in personality such as depression, irritability, and psychosis.he main syndrome of vitamin B 12 deficiency is pernicious anemia. It is characterized by a triad of symptoms: 1  Anemia with bone marrow promegaloblastosis (megaloblastic anemia). 2  This is due to the inhibition of DNA synthesis (specifically purines and thymidine).