High dose vitamin B12 supplements can sometimes cause side effects, including headache, nausea, vomiting, mild diarrhea, itching, skin rash, tiredness, weakness, or tingling in the hands and feet. OC: Why Do I Feel Worse After My B12 Injection? Welzo Taking Vitamin B12? Read this first. While B12 is often hailed as a safe energy and brain booster, many real users are reporting unexpected side effects including anxiety, insomnia, Lipo B Skinny Shot Injections SpaDerma Med Spa Why did B12 make me feel worse?, I hear this all the timewomen tell me they started B12 shots or IVs and suddenly felt overstimulated, jittery, or couldnt sleep., Heres whats really going on: