Why Do I Feel Worse After My B12 Injection? Understanding Post Injecti Oxford Origins Why Do I Feel Worse After My B12 Injection? Causes & Management Bolt Pharmacy Some Types Of Vitamin B12 Supplements Can Cause This Side Effect Experiencing any of these symptoms? B12 supplementation might be right for you. Some causes of low B12 are diet, alcohol use, certain diseases like Chrohn's and Celiac Disease, and Gastric Bypass surgery, Vitamin B12 Injection Side Effects and Safety 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