The proper dose for a B12 injection depends on the type of health problem and a person's health condition. A patient gets an injection once a day for the first 6 to 7 days. If the condition starts improving, the dose is one injection a day for 2 weeks, and then one injection every 3 to 4 days for 2 to 3 weeks, notes the United States National Institutes of Health's MedlinePlus.
Patients suffering from pernicious anemia may have to get the injection for the rest of their lives, notes Drugs.com. Also known as cyanocobalamin, B12 injection is a man-made form of vitamin B12 used to treat vitamin B12 deficiency. This vitamin helps the body to make use of carbohydrates and fats to make proteins and get energy.