how do you inject a b12 shot to Give Injection: Step-By-Step Instructions B12 Shots vs. IV Infusions:
Description
Liblau, et al
Evidence reviews for risk factors and symptoms and signs for vitamin B12 deficiency: Vitamin B12 deficiency in over 16s: diagnosis and management: Evidence review B

Such developments would provide researchers with an expanded toolkit for investigating the complex relationships between metabolism, aging, and disease, potentially accelerating discoveries across multiple fields of biomedical research

Many clients start with weekly injections for 4-6 weeks, then transition to bi-weekly or monthly maintenance

But these arent all of the medical reasons and there are also medications that interfere with B12 absorption
