where should a b12 injection be given How to self-inject intramuscular vitamin - Overview B12 Shots vs. IV Infusions:

Hong S et al

The duration of the effects varies by individual, depending on their health needs and lifestyles

Where possible, avoid suddenly reducing the B 12 dose or injection frequency, as too rapid dose reduction may worsen symptoms or lead to symptom recurrence.26 Instead, gradually increase the interval between B 12 injections over the course of several months while monitoring symptoms

But B12 supplements don't seem to have any effect on people already at normal levels

However, needles and syringes can be a cause of concern for many people, especially if needing to be self-administered or administered to a loved one