side effects b12 injections Shots: Vitamin B12 Benefits, Uses, Risks How Quickly Does B12 Shot
Description
Starting doses on the lower end (300mcg) and progressing slowly makes sense

Easily absorbed by the animal for for quick utilization to help maintain both nerve and blood cells

PMID: 28189172 6 Intramuscular Cyanocobalamin Treatment in Patients with Corpus Atrophic Gastritis and Vitamin B12 Deficiency: Efficacy and Predictors of Increased Requirement-A Monocentric Longitudinal Real-Life Cohort Study

I guess it depends on how low each persons individual levels are and how quickly they utilise the B12

Many of our clients report feeling a wave of relaxation washing over them within minutes of starting their infusion
