b12 injection feeling tired I Feel All the Time. Do I Need a Shot?: Shawn Veiseh, M.D.: Family Medicine Chronic Fatigue: Can B12 Shots
Description
Older people, those with certain health issues, and those with diet problems might need them more often

Fortunately, B12 supplementation can help to improve these symptoms, with improvement typically seen within days to weeks
Can I continue my multivitamin or other supplements while on this injection

(PubMed) Potter K, Hankey GJ, Green DJ, Eikelboom J, Jamrozik K, Arnolda LF
Clinical Nutrition, 38(4), 1747-1755