What are the side effects of vitamin B12 injections?

Side effects of vitamin B-12 injections include an overall feeling of puffiness, fatigue, itching, rash, hives, and muscle weakness, cramps or pain, states MedlinePlus. Reactions limited to specific body parts include leg pain, headache, thirst, swelling of the limbs and red face.

Other side effects that show up in certain body systems are frequent urination, diarrhea, confusion, dizziness, shortness of breath, coughing, wheezing and difficulty breathing or swallowing, according to MedlinePlus. B-12 injections are prescribed for pernicious anemia, vegan diets and conditions that reduce the body's ability to absorb the vitamin from food. Over time, vitamin B-12 deficiency leads to anemia and permanent nerve damage.