can u inject too much b12 Are Vitamin Shots Right for You? Too Much Vitamin B12: Risks,
Description
Link for how to break an ampoule and load syringe

Individual sensitivity Some people are more prone to post-injection headaches
If your dog is suffering from inherited selective cobalamin malabsorption, chronic enteropathies, or EPI, your vet may recommend 250 to 1,000 mcg of cobalamin, taken orally once a day

This is likely because the B12 is helping your blood cells metabolize energy more efficiently

Baseline Health Assessment : Including cardiovascular evaluation, metabolic panel, and hormone level assessment to establish pre-study baselines