The main side effects of a vitamin B12 injection include redness at the injection site, mild diarrhea, itching or a feeling of swelling all over the body, according to MedicineNet. If these side effects persist or get worse, contact the doctor or pharmacist immediately. An injection of vitamin B12 may cause low potassium levels in the blood, a condition known as hypokalemia.
Cyanocobalamin, a synthetic form of vitamin B12 used for injections, may cause serious side effects, such as muscle cramps, weakness, chest pain, sudden vision changes and slurred speech. MedicineNet explains allergic reactions to this injection are rare. The Mayo Clinic reveals injections may interfere with some medications. Injectable vitamin B12 is "likely safe" for patients when used properly, according to WebMD.
Vitamin B12 shots are most commonly used to treat pernicious anemia and a vitamin B12 deficiency, according to the Mayo Clinic. These shots are given into a muscle or underneath the skin, and dosage amounts are determined by a doctor with regard to the medical condition and response to treatment, according to MedicineNet.
The Mayo Clinic explains vitamin B12 shots should not be used for weight loss even though some weight loss programs tout vitamin B12 injections as a way to boost metabolism. Usually, patients deficient in this nutrient see an energy boost from vitamin B12 injections.