are there any side effects to b12 injections of Injection (Common and Serious) B12 Shot Guide: Benefits, Dosage
Description
This helps to force the liquid vitamin into the syringe
If the goal is scale weight, this is not the right tool

Nicotinamide N-methyltransferase knockdown protects against diet-induced obesity. Nature Medicine , 20(4), 398-403

YES NO May be one of the cause, but if pain persists contact treating Physician
