The Office of Dietary Supplements of National Institutes of Health gives fact sheets about certain nutrients, including vitamin B12, in which they clarify that while generally considered safe, there are certain concerns regarding the dosage, form, and stability of the injectable version of vitamin B12 as compared to oral supplements
The abdominal region contains two distinct types of adipose tissue
Roughly 85% to 90% of a therapeutic dose undergoes Phase II conjugation (glucuronidation via UGT1A1, UGT1A6, and UGT1A9, plus sulfation via SULT1A1), with only about 5% to 10% oxidized by CYP2E1 into the reactive metabolite NAPQI, per Hodgman and Garrard's pharmacology review
PMID: 25019414
Comparison: the four peptides, side by side I dose both blends subcutaneously, abdomen or thigh, once daily for the first three to four weeks postoperatively