Vitamin B12 is scientifically used to support or treat "wasting" (also known as cachexia or muscle wasting), particularly when the wasting is linked to underlying vitamin B12 deficiency. Wasting is often seen in chronic illnesses such as cancer, HIV/AIDS, and severe malnutrition, where appetite, nutrient absorption, and muscle mass are reduced. Vitamin B12 plays a crucial role in DNA synthesis, red blood cell formation, and neurological function. Its deficiency can lead to anemia, weakness, and neurological symptoms, which can contribute to or exacerbate wasting syndromes.

Several clinical guidelines recommend vitamin B12 supplementation in patients with confirmed deficiency, particularly in populations at risk for malnutrition or absorption issues (e.g., elderly, those with gastrointestinal disorders). Studies have shown that correcting B12 deficiency can improve hematological parameters, energy, and sometimes appetite, which may indirectly help prevent further wasting. However, there is limited direct evidence that B12 supplementation alone reverses or significantly treats established wasting in the absence of deficiency. Meta-analyses and randomized controlled trials on cachexia management generally focus on multimodal approaches, including nutritional support, exercise, and sometimes anabolic agents.

In summary, vitamin B12 is scientifically justified for treating wasting only when a deficiency is present, with moderate evidence (rating 3/5) supporting its use for this indication. It is not a standalone therapy for all causes of wasting but is important in the context of deficiency correction.