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Although cobalamin (vitamin B 12 ) deficiency was described over a century ago, it is still difficult to establish the correct diagnosis and prescribe the right treatment. Symptoms related to vitamin B 12 deficiency may be diverse and vary from neurologic to psychiatric. A number of individuals with vitamin B 12 deficiency may present with the classic megaloblastic anemia. In clinical practice, many cases of vitamin B 12 deficiency are overlooked or sometimes even misdiagnosed. In this review, we describe the heterogeneous disease spectrum of patients with vitamin B 12 deficiency in whom the diagnosis was either based on low serum B 12 levels, elevated biomarkers like methylmalonic acid and/or homocysteine, or the improvement of clinical symptoms after the institution of parenteral vitamin B 12 therapy. We discuss the possible clinical signs and symptoms of patients with B 12 deficiency and the various pitfalls of diagnosis and treatment.