Symptoms Signs of Vitamin B12 Deficiency: You Shouldn't Ignore Medinaz Vitamin B12 Deficiency Vitamin B12 deficiency happens when the body does not have enough vitamin B12. Vitamin B12 is important for making healthy red blood cells and 3 Rare But Serious Side Effects of Vitamin B12 Supplements B12 deficiency can damage your nervous system with a completely normal blood count. In a landmark NEJM study, 28% of patients with neuropsychiatric B12 damage had no anemia or macrocytosis. B12 runs How Vitamin B12 Injections Affect Your Body: Good or Bad?