adverse reaction b12 injection reactions to vitamin injections due... : Allergy Vitamin B12 Side Effects: Risks

Vitamin injections are generally well tolerated when they are medically appropriate and administered correctly

Cell Host Microbe 19 , 455469 (2016)

The proposed mechanisms include upregulation of beta-3 adrenergic receptors in adipose tissue, activation of the cAMP-PKA lipolytic cascade through enhanced catecholamine sensitivity, direct or indirect inhibition of acetyl-CoA carboxylase (ACC) reducing de novo lipogenesis by approximately 43-57% in vitro, and activation of hormone-sensitive lipase (HSL) with increased glycerol release

Rodent studies showed dose-dependent reductions in body fat mass particularly visceral fat without changes in lean mass, glucose tolerance, or linear growth, enabling cleaner metabolic research designs than full GH protocols

Your body might not be absorbing the vitamin at a steady rate when you are taking supplements, and thats where a Vitamin B12 shot can help accelerate the process