A 50 mg vial mixed with 3.0 mL of bacteriostatic water gives about 16.7 mg/mL, and the example subcutaneous range is 2.5 to 5 mg per day
Once you know the concentration in mg/mL, a dose in mg becomes a volume in mL by simple division, and that volume becomes insulin-syringe units by multiplying by 100 (because a U-100 syringe is marked so that 1 mL = 100 units)
Yes, people with pernicious anemia need vitamin B12 for life
Staying current with regulatory updates is advisable
Below is a clinicians view of who actually needs B12 injections, how often, what the NHS will and will not do for you, and what we check before we book the first shot
As you can see most of the causes are a result of not being able to absorb B12 in the first place and not actually from intake itself