Constantly tired? It might not just be stress. Sometimes the smallest things can make the biggest difference in how you feel day to day. Save this post if you've been feeling off The Benefits Of Vitamin B12 Ugly London. Acupuncture & Chinese Herbal Medicine When Do You Feel Less Tired After B12 Shot? I Feel Tired All the Time. Do I Need a B12 Shot?: Shawn Veiseh, M.D.: Family Medicine 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.