If the available vial shows Vitamin B12 injection 1000 mcg, confirm whether the wording refers to the amount per mL or the amount in a dose
Any sort of GI problem (including hormone imbalances like thyroid problems) may limit absorption
There are large vessels and nerves in this area and this area should really only be used if there are no other options
Known deficiency, prior low B12, fatigue follow-up, diet-related risk, and histories that can affect absorption are the most common reasons patients ask for B12 injections
Even if the vial is brand new, you should always clean the stopper
Beyond the temperature-cycling concern (refrigerator doors experience wider temperature swings than interior shelves), the door is exposed to room lighting every time the refrigerator is opened