b12 injected into blood vessel Treating Severe Vitamin Deficiency B12 Shots vs. IV Infusions:

Unlike other vitamins, the absorption of vitamin B12 depends on a protein produced in the stomach called intrinsic factor

The fragment does not bind the growth hormone receptor or activate the JAK2-STAT5 signaling pathway

And here's where it get difficult again...there are many types of anaemia (not just macrocytic or iron anaemia) so pinning the anaemia down to just those causes (especially if those potential causes are being treated) might be a reductive approach - especially as you still have symptoms which could be due to anaemia

Treating hypocobalaminemia (B12 deficiency)

Some peptides, mainly those not FDA-approved, may be obtained from compounding pharmacies or unverified sources