Introduction: In this article, seven questions answered about vitamin B12 deficiency. We will discusses the absorption of vitamin B12, the physiology, the importance and prevalence of the deficit, the way to recognize it early, and then we will talk about the several laboratory biomarkers to detect vitamin B12 deficiency, and the treatment.Dorothy Hodgkin and her collaborators had discovered the entire structure of vitamin B12 by means of X-ray crystallographic methods. They have showed that the vitamin was a cobalt-containing, cyanolated, amidated tetrapyrrole. The location of the cobalt is in the center of a ring-contracted modified tetrapyrrole macrocycle, coordinated via the four pyrrole nitrogen atoms. The cobalamin tetrapyrrole ring, exclusive of cobalt and other sidechains, is called a corrin. All of the compounds which contain this corrin nucleus are called corrinoids. Cobalamins is different in the nature from additional side groups bound to cobalt, like hydroxyl (Hydroxycobalamin-H- Cbl), deoxy-5′- adenosine (Deoxy-5′- adenosylcobalamin- Ado- Cbl), and cyanide (Cyanocobalamin Cn-Cbl), Methyl (methylcobalamin-Me- Cbl). The term vitamin B12 is used to identify the various forms of cobalamin, like Me-Cbl and Ado-Cbl, and they are also identified as complete corrinoids. Me-Cbl and Ado-Cbl are the active forms of vitamins that are used as coenzymes in the human cells.2 Vitamin B12 is needed in these coenzyme forms to convert L-methylmalonyl CoA to succinyl-CoA and homocysteine to mehionine. These pathways are important in the metabolism of branched-chain amino acids and fatty acid and to regenerate the methyl donor S-adenosylmethionine. Its malfunction causes a shortage, affecting DNA synthesis and the physiological processes such as hematopoietic process of the red blood cells Aim of work: In this review, we will discuss Vitamin B 12 causes and treatmentsMethodology: We did a systematic search for Vitamin B 12 causes and treatments using PubMed search engine (http://www.ncbi.nlm.nih.gov/) and Google Scholar search engine (https://scholar.google.com). All relevant studies were retrieved and discussed. We only included full articles.Conclusions: Vitamin B12 deficiency is a widespread disease and has serious irreversible clinical complications. There is no single widespread agreed diagnostic biomarker for vitamin B12 deficiency. Additional studies are required to define such standard marker, and to check whether positivity of APCA or AIFA can indirectly confirm the difficiency.The Clinical Laboratory plays a basic role in vitamin B12 deficiency. It should design and lead active screening strategies to increase its detection before the clinical symptoms arise, identify cases of autoimmune disease, and promote prompt treatment after abnormal serological tests.