Vitamin B-12 injections only increase energy in people who are vitamin B-12-deficient, according to the National Institutes of Health. The vitamin is in most animal-derived foods naturally, and it is added to many nutritionally-fortified foods. Adults need about 2.4 micrograms of vitamin B-12 per day, states WebMD.
Physicians administer vitamin B-12 shots to patients with pernicious anemia because their systems are unable to obtain the vitamin from food sources, explains the National Institutes of Health. Vegans and vegetarians may develop vitamin B-12 deficiencies unless they take supplements or eat vitamin B-12-fortified foods. Symptoms of vitamin B-12 deficiency include confusion, fatigue, weight loss, tingling and numbness of the extremities, and depression.