It all boils down to the need of the patient, even when the injectables are good but oral supplements are also acceptable depending on the needs of the patient
Kannt A, Pfenninger A, Teichert L, Tnjes A, Dietrich A, Schn MR, Klting N, Blher M. Association of nicotinamide-N-methyltransferase mRNA expression in human adipose tissue and the plasma concentration of its product, 1-methylnicotinamide, with insulin resistance. Diabetologia. 2015 Apr;58(4):799-808. doi: 10.1007/s00125-014-3490-7. Epub 2015 Jan 18. PMID: 25596852; PMCID: PMC4351435
Any literature I've seen only mentions the liver
Neuropathy due to vitamin B12 deficiency primarily affects the posterior columns of the spinal cord, and as it advances, can involve large-diameter myelinated fibers (A) in the peripheral nerves [12]