Feeling worse after B12 Injection: Answering concerns. La Vie Med Spa I Feel Tired All the Time. Do I Need a B12 Shot?: Shawn Veiseh, M.D.: Family Medicine When Do You Feel Less Tired After B12 Shot? Feeling Tired All the Time? It Might Be a Vitamin B12 Deficiency Fusion HealthCare Is it too soon to administer another vitamin B12 (cyanocobalamin) injection to a patient with a vitamin B12 level of 308 pg mL who received an injection 6 days ago for fatigue? Vitamin B12 Injection: Benefits, Uses and Who Needs It Periville Pharmacy