All medications have side effects. These side effects may vary based on the medication dosages and each person’s sensitivity to the drug ingredients. Side effects due to an allergic reaction to B12 include hives, itching, swelling of your face, lips, tongue or throat, or difficulty breathing. Get emergency help.
Additional side effects from B12 injections may include abdominal or stomach pain, bleeding from the gums or nose, blue lips or fingernails, chest pain, cough, coughing up pink, frothy sputum, decreased urine output, difficulty breathing, difficulty swallowing, dilated neck veins, dizziness, fatigue, eye pain, fast heartbeat, headache, increased sweating, irregular breathing or heartbeat, puffiness or swelling of the face, fingers, feet or lower legs, tightness in the chest, or weight gain. Nausea, vomiting, diarrhea, loss of appetite, or rashes may also occur.
B12 injections can cause low potassium levels. Symptoms of low potassium include muscle cramps, weakness, and irregular heartbeat. Contact a health care provider whenever you experience a side-effect that is severe, concerning, or persistent.