Functional vitamin B12 deficiency is a syndrome where a wide variety of symptoms in the presence of “normal” serum levels of the vitamin respond to vitamin B12 therapy. A series of patients with functional vitamin B12 deficiency are described whose presenting features were drenching night sweats and fatigue. Reliance on serum vitamin B12 levels as a diagnostic test would have obscured the cause of their symptoms. Serum homocysteine and/or methlymalonic acid levels should be done in all patients with suspected B12 deficiency. Normal levels of these metabolites do not exclude diagnosis and empirical treatment may be justifiable in certain cases. Author also argues that oral vitamin B12 treatment results in suboptimal clinical response in a vast majority of patients and intramuscular route should be preferred in most patients.