For years, doctors have used B-12 injections to treat chronic fatigue. Because the body has a very difficult time absorbing B-12 in large quantities through the digestive system, supplementation with traditional tablets is often ineffective. Sublingual B-12 is a special formulation that is dissolved under the tongue, meaning that the vitamins can be absorbed directly into the blood stream. Studies have shown that this is an effective way to increase the B-12 levels in the blood.