Lack of energy is a common symptom of anemia, which is a condition characterized by a decrease in the number of red blood cells or hemoglobin in the body. Anemia can be caused by a variety of factors, including nutritional deficiencies, chronic diseases, and certain medications. Symptoms of anemia include fatigue, pale skin, shortness of breath, dizziness, and headaches. Treatment for anemia typically involves dietary changes, iron supplements, and other medications.