How Long Does It Take for Vitamin B12 to Work? Simple Guide We are now offering Vitamin B12 Injections So, why should you get vitamin B12 injections? Enhanced Absorption: Injections ensure direct absorption into the bloodstream, bypassing potential absorption issues in the digestive tract, Does Vitamin B 12 Injections Work For Weight Loss? Vitamin B12 shots are injected intramuscularly bypassing the digestive process,which means they start to work immediately.Typically my clients will experience positive effects between 24 72 hours Do You Need a Prescription for B12 Injections? OmegaQuant