B12 Shots: Vitamin B12 Injections Benefits, Uses, Risks Dr. Axe B12 Injection Dosage and Frequency: 7 Essential Guidelines for Adults Liv Hospital 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: 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: How Quickly Does a B12 Shot Work? Benefits & What to Expect Are B12 Injections Safe? Should I Get A B12 Shot?