Vitamin B12 shots are designed for people who have a deficiency in vitamin B12. According to Dr. Roy Benaroch on KevinMD.com, anemia, poor diet, depression, neuropathy and irritability are all causes of a deficiency in vitamin B12.
According to Dr. Benaroch, a blood test can determine the amount of vitamin B12 in the body; however, the levels in the blood do not always determine if there is sufficient B12 in the actual cells. Other tests that include homocysteine levels and methylmalonic acid help determine how much B12 is in the cells. Once adults experience fatigue, they often assume it is because of a lack of B12, but at times, the fatigue can be a result of too much caffeine, depression or sleep apnea.
Even though vitamin B12 deficiency can be treated by the pill ORAL B12, many people prefer taking an injection instead of a pill. For example, if a person had a serious abdominal surgery, taking an injection is more beneficial. In addition, Dr. Benaroch reports that patients also prefer taking a B12 shot instead of a B12 pill because it seems more effective in general. An injection instantly hits the bloodstream, while a pill takes time to be digested.