Although rare, allergic reactions to vitamin B12 can occur and should be treated as a serious side effect. Symptoms may include hives, difficulty breathing, or swelling of the face, lips, tongue, or throat. In severe cases, an individual may experience anaphylaxis, a life-threatening reaction that requires immediate medical attention.
Sensitivities to cobalt, a key component of vitamin B12, are believed to be the primary cause of these allergic responses. If any of these symptoms develop, seek emergency care without delay.

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