Can having too much B12 cause nausea and vomiting? What Should You Know About Vitamin B12 Injections? Happy Hands Aesthetics 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 Lets talk about B12 because yes, you can overdose on it. Most people think B12 is something you can take endlessly because the extra just gets flushed out., Not true., B12 toxicity is real and What Happens If You Inject B12 Wrong: 6 Red Flags