Why Do I Feel Worse After My B12 Injection: 6 Body Reactions Vitamin B12 deficiency can cause symptoms like headaches, fatigue, depressive mood, gastrointestinal issues, and more. While vegetarians are at a higher risk of vitamin B12 deficiency, it can affect non vegetarians too. Check B12 Headache: The Link Between Vitamin B12 and Headaches B12 Injections: What's the Buzz? Vitamin B12 is the star of several essential functions in the body. However, it's common to be deficient in B12 and as a consequence experience fatigue,