A Vitamin B12 injection is a shot of Cyanocobalamin, a man-made form of Vitamin B12 as reported by Drugs.com. Cyanocobalamin is injected straight into the blood stream and is used to treat Vitamin B12 deficiency.
The National Institutes of Health indicates that Vitamin B12 deficiency may be caused by conditions such as pernicious anemia, malabsorption of the vitamin due to certain diseases, infections or medications, or a vegan diet. B12 deficiency can cause anemia and permanent damage to the nerves.
Vitamin B12 injections are also commonly used in weight loss clinics as weight-loss aids. While some claim that the injections provide extra energy and boost metabolism, the Mayo Clinic does not agree. There is no evidence that B12 injections aid weight loss, and unless a person is B12 deficient, the injections are not likely to provide any additional energy.