Itching, moderate diarrhea, pain/redness at the injection site, or a generalized feeling of swelling of the body are possible side effects. Inform your doctor or pharmacist as soon as possible if any of these side effects persist or get worse. B12 can be administered intramuscularly or subcutaneously at doses of 100 mcg every day for a week, every other day for another week, then every three to four days for a month. After that, 100 mcg should be injected monthly for the rest of your life. In some patients with poor vitamin B12 absorption, B12 injections may be superior to B12 tablets. The majority of patients respond just as well to pills as they do to injections.