vitamin d3 and b12 injection D-3 Hydroxocobalamin Injection: Vitamin B12 for
Description
8 weeks 2 x 10 mL bottles 5 vials use 15 mL total

This is a serious problem, because doctors often agree to prescribe only a single shot every few months

Online communities promoting these compounds together lack medical or scientific oversight, and anecdotal reports should not be treated as evidence of safety or efficacy
Yes, when supervised
Associations between homocysteine, folic acid, vitamin B12 and Alzheimer's disease: insights from meta-analyses

This is due to the presence of intrinsic factor, a glycoprotein in the digestive tract that limits the bioavailability of B12