Vitamin B12 Injections are very popular for treating a B12 deficiency, and for boosting energy levels. Vitamin B12 is a nutrient that helps to keep your nerves and blood cells healthy. Deficiency is common, particularly in elderly people as the ability to absorb B12 can decrease with age. Some symptoms of a deficiency include lack of energy, insomnia, increased levels of depression and anxiety, loss of appetite, muscle weakness and yellow eyes. The main benefits of Vitamin B12 include improvement in energy levels and concentration, better sleep, improved moods and boosts metabolism.