Why Do I Feel Worse After My B12 Injection? Welzo Side Effects of B12 Injection (Common and Serious) Credihealth Now offering B12 injections! If these symptoms sound all too familiar, it might be time to check your vitamin levels. B12 injections may be absorbed up to 90% more effectively than oral B12 Injections for Energy: How do they work? RunToTheFinish B12 Shots Dosage for Adults: How Much, How Often, and Key Injection Guidelines Liv Hospital Feeling constantly tired, foggy, or run down? Vitamin B12 plays a key role in energy production and nervous system health and low levels can lead to fatigue, brain fog, and weakness.