Does Vitamin B12 Make You Gain Weight?

UIG via Getty Images/Universal Images Group/Getty Images reports that rapid weight gain is one of the common side effects of vitamin B12 injections. Swelling, diarrhea and a mild rash are also side effects that people commonly experience when they have a vitamin B12 injection. Doctors administer these injections to treat patients whose vitamin B12 deficiency has led to pernicious anemia or another medical condition.