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Chronic Fatigue Syndrome

Medically reviewed by Last updated on Aug 31, 2021.

What is Chronic Fatigue Syndrome?

Harvard Health Publishing

Chronic fatigue syndrome, also known as myalgic encephalomyelitis, is a complicated illness characterized by at least six months of extreme fatigue that is not relieved by rest, and a group of additional symptoms that also are constant for at least six months. In many people with chronic fatigue syndrome, the disorder begins suddenly, often following a flulike infection or an episode of physical or psychological trauma, such as surgery, a traumatic accident or the death of a loved one. In other cases, chronic fatigue syndrome develops gradually. The illness lasts for many months or years, and only a small percentage of people recover full health.

Many people feel tired a lot of the time, and many seek help from their doctors. Most people who experience chronic (long-lasting) fatigue are not suffering from chronic fatigue syndrome. Depression and overwork are much more common causes of chronic fatigue.

The exact cause of chronic fatigue syndrome remains a mystery. The illness can follow a number of common infectious illnesses, such as Lyme disease or infectious mononucleosis, but not all cases are tied to infections. Testing has found that people with chronic fatigue syndrome have abnormalities in the brain, particularly in the hypothalamus (a part of the brain that regulates hormones and vital functions) and the pituitary gland. Testing also has found that patients have abnormalities in the part of the nervous system called the autonomic nervous system, which controls blood pressure, heart rate, body temperature and other vital body functions. For example, many patients with chronic fatigue syndrome have an unusually high heart rate and low blood pressure when they have been standing for a while.

Several parts of the immune system remain activated for long periods in people with chronic fatigue syndrome. There is growing evidence that some patients with chronic fatigue syndrome have an autoimmune condition: their immune system is attacking particular tissues in the body.

Patients with chronic fatigue syndrome have defects in the ability of cells in their bodies to make energy. Some studies indicate that certain genes are built differently, and that the activity of genes in white blood cells is different, in patients with chronic fatigue syndrome.

Many different tests of the brain, and of the autonomic nervous system, reveal abnormalities that are not seen in healthy people of the same age, or in people with other conditions that can cause fatigue, such as depression.

Many of the abnormalities of the immune system, energy metabolism, and the nervous system seem to come and go. Furthermore, not all of the abnormalities affect every patient with chronic fatigue syndrome.

In the United States, federal health authorities estimate that chronic fatigue syndrome affects 1 to 8 of every 1,000 Americans older than age 18. Women are affected about twice as often as men. Although the illness is most common in people 25 to 45 years old, chronic fatigue syndrome can attack people of all age groups, including children. The condition also is found in people of all racial, ethnic and economic backgrounds. It appears to be more common in African-Americans and Latinos, and in people in lower socioeconomic groups. It appears to be less common in Asian-Americans. Studies from the U.S. Centers for Disease Control and Prevention (CDC) and other research groups estimate that the United States loses between $9 to $25 billion each year in reduced productivity and medical expenses due to chronic fatigue syndrome.

Although most cases of chronic fatigue syndrome do not occur during epidemics, at least 30 outbreaks of chronic fatigue syndrome have been reported, during which many people in the same area suddenly developed the illness at the same time. However, health experts have failed to identify a cause for their chronic fatigue syndrome symptoms.


The most prominent symptom of chronic fatigue syndrome is an unexplained feeling of fatigue, which is not relieved by rest. This fatigue is severe enough to decrease a person's activity level at home, work or school by 50% or more. In addition, the diagnosis requires that patients should have at least four of the following symptoms that also are present for at least six months:

  • Impaired concentration or short-term memory, severe enough to affect routine activities at home, work, school or social functions
  • Sore throat
  • Enlarged lymph nodes (swollen glands) in the neck or underarm area
  • Muscle pain
  • Pain in several joints, with no redness or swelling
  • Headaches that are different in some way: a new type of headache pain, a new pattern of headaches or headaches that are more severe than before
  • Sleep that doesn't refresh, or not feeling rested on waking
  • An extreme reaction to exertion: feeling sick after exercise or strenuous activity, often not starting until the next day

People with chronic fatigue syndrome often have other symptoms that are not part of the official definition of the illness, such as nausea and difficulty tolerating alcoholic beverages or medicines that act on the brain. Many people also have allergies, such as hay fever (allergic rhinitis) or recurring sinus problems.

About half of people with chronic fatigue syndrome develop depression in the months and years after their illness starts. However, available evidence indicates that chronic fatigue syndrome is not a psychiatric illness. Rather, it appears to be a physical illness that leads to depression in some people.


Although there is a lot of evidence that chronic fatigue syndrome is caused by a physical problem involving the immune system, energy metabolism and the nervous system, there is no laboratory test or procedure to confirm the diagnosis. Until a better way is found, doctors must diagnose chronic fatigue syndrome based on whether a person has the symptoms of the illness and by eliminating other illnesses that can cause long-lasting fatigue.

For this reason, your doctor will ask about symptoms of other fatigue-causing illnesses including:

  • Hypothyroidism (underactive thyroid gland)
  • Adrenal insufficiency (underactive adrenal gland)
  • Heart disorders
  • Sleep apnea or narcolepsy
  • Side effects of medications
  • Cancer
  • Hepatitis B or hepatitis C
  • Certain psychiatric illnesses, especially major depression, bipolar disorder, schizophrenia and delusional disorders and dementia
  • The eating disorders anorexia nervosa and bulimia
  • Drug abuse, including alcohol abuse
  • Severe obesity

Your doctor will examine you and assess your mental status. Some basic blood tests may be ordered, such as a red blood cell count (hematocrit); white blood cell count and differential white blood cell count, thyroid, kidney and liver tests. Additional, more specialized testing may be needed, including testing of your autonomic nervous system.

Expected Duration

To be diagnosed as chronic fatigue syndrome, symptoms must last for at least six months. Unfortunately, in many people, symptoms persist for years. Symptoms tend to be worst in the first one to two years, and most people's level of functioning gradually improves over time. However, only a small percentage of people recover to full health.


Because the cause of chronic fatigue syndrome remains unknown, there is no way to prevent it.


There is no proven treatment for chronic fatigue syndrome. Both gradual aerobic exercise programs and cognitive behavioral therapy — counseling designed to change beliefs about the condition — improve level of function, but neither cures the illness. In patients with a similar condition, fibromyalgia, low doses of tricyclic drugs have been shown to improve symptoms, probably by improving a sleep disorder that is part of the illness. No one approach is best for everyone with chronic fatigue syndrome, and the condition rarely is cured.

In general, doctors use a combination of the following:

  • Lifestyle changes. Patients are often initially encouraged to slow down, minimize physical exertion and try to avoid psychological stress. They learn to save their energy for essential activities at home or work and to cut back on less-important activities.
  • Resuming exercise gradually but steadily. With the help of a physical therapist, patients begin an exercise program in which aerobic physical activity begins very slowly, and is increased very gradually. Patients can expect occasionally to feel worse the day after aerobic exercise. If that happens, many experts recommend avoiding exercise for several days and then resuming a less-intensive program, and slowly increasing the pace.
  • Treating existing psychiatric problems. In the approximately 50% to 60% of people with chronic fatigue syndrome who develop depression, antidepressant treatment and talk therapy can be valuable in treating the depression; however, the chronic fatigue is rarely if ever cured by antidepressant therapy.
  • Treating existing pain. Aspirin, acetaminophen (Tylenol) or nonsteroidal anti-inflammatory drugs (NSAIDs) are used to treat headaches, muscle pain and joint pain. Antidepressant medications also may help to reduce chronic pain.
  • Treating existing allergy symptoms. Antihistamines and decongestants are used to treat allergy symptoms.
  • Experimental therapies. Some studies indicate that high doses of omega-3 fatty acids (such as fish oil capsules) may be helpful. Several anti-viral therapies are being tested. Stimulants are sometimes prescribed, but their value has not been carefully tested.

Treatment options

The following list of medications are in some way related to or used in the treatment of this condition.

When To Call a Professional

Call your doctor if you have symptoms of chronic fatigue syndrome, especially if extreme fatigue prevents you from fully participating in activities at home, work or school.


People with chronic fatigue syndrome usually experience their most severe symptoms in the first one to two years of illness. After that time, a small number of people recover totally, and a smaller number become totally incapacitated. For most people, there is gradual improvement, although they usually do not achieve the level of activity they were capable of before becoming ill. Recovery tends to be less likely among people who:

  • Have symptoms for a longer time
  • Have long-standing depression
  • Are older than 40 when symptoms start
  • Have multiple physical symptoms

Learn more about Chronic Fatigue Syndrome

Treatment options

Care guides guides (external)

External resources

Centers for Disease Control and Prevention (CDC)

Medline Plus, from the National Institutes of Health/National Library of Medicine

International Association for Chronic Fatigue Syndrome/Myalgic Encephalomyelitis

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.