People take vitamin B12 injections due to their inability to absorb the essential nutrient from foods or dietary supplements. An estimated 1.5 to 15 percent of Americans suffer from vitamin B12 deficiency, reports the United States' National Institutes of Health.
The difficulty in absorbing vitamin B12 is commonly attributed to a medical condition known as pernicious anemia. This disorder prevents the stomach from producing a protein called intrinsic factor, which is necessary for the absorption of the vitamin into the bloodstream. Patients diagnosed with this disease are treated using vitamin B12 shots. However, health professionals may also prescribe high doses of oral medication.
People may also opt for vitamin B12 injections due to old age or other digestive-related problems, such as Crohn's disease and celiac disease. Untreated vitamin B12 deficiency may result in fatigue, appetite loss, constipation, megaloblastic anemia, weight loss and certain neural problems, explains NIH.