In the 50s, when chemotherapy wasnt yet available, massive doses of B12 shots were given to children with a neuroblastoma tumor every two days, for 2-3 years with great success: NEUROBLASTOMA: AN EVALUATION OF ITS NATURAL HISTORY AND EFFECTS OF THERAPY, WITH PARTICULAR REFERENCE TO TREATMENT BY MASSIVE DOSES OF VITAMIN B12
Theres no single right answer here
It also depends on your general health and how your body processes the vitamin
its administered via injections or capsules in clinical settings.[4] It enhances fat burning, energy, insulin sensitivity, muscle preservation, and repair, while indirectly boosting NAD+ by slowing depletion.[5][23][24] Its membrane-permeable nature allows efficient cellular entry, making it suitable for therapeutic use
No, it is not FDA-approved, and much of the evidence is preclinical