Vitamin B12 is an essential nutrient involved in neurological function and the synthesis of DNA and red blood cells. Its use in dementia, particularly in the elderly, is supported by scientific evidence primarily in the context of deficiency states. Vitamin B12 deficiency can cause reversible cognitive impairment, which may mimic or exacerbate dementia syndromes such as Alzheimer's disease. Correction of the deficiency has been shown to improve cognitive symptoms in some cases, especially when diagnosed early. However, large randomized controlled trials in patients with established dementia and normal B12 levels have not demonstrated significant benefits from supplementation. Therefore, while Vitamin B12 supplementation is scientifically validated for treating cognitive impairment due to deficiency, it is not broadly effective as a treatment for dementia of other causes or when deficiency is absent. Clinical guidelines recommend screening for B12 deficiency in patients presenting with cognitive symptoms due to the potential for reversible dementia. Thus, the overall evidence supporting Vitamin B12 for dementia is moderate and highly specific to cases of deficiency.