Does Vitamin B12 Make You Gain Weight?

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Drugs.com 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.

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