Chronic fatigue syndrome (CFS) is a long-term condition that causes people to feel extremely tired and weak. They may also experience headaches, dizziness, or other physical symptoms. They may also exhibit emotional symptoms such as anger or sadness. The symptoms of CFS differ from person to person. Many CFS symptoms are similar to those of other medical conditions such as mono, Lyme disease, or depression. Symptoms can change over time, even within the same person. This makes treating the illness more difficult. There is no single medication or treatment that can address all of the possible symptoms.