Merck Manual

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Michael R. Wasserman

, MD, California Association of Long Term Care Medicine

Reviewed/Revised Feb 2023
Topic Resources

Fatigue is when a person feels a strong need to rest and has so little energy that starting and sustaining activity is difficult.

Fatigue is normal after physical exertion, prolonged stress, and sleep deprivation. However, fatigue that increases and develops after activities that previously did not cause it may be one of the symptoms, or, occasionally, the first symptom of a disorder.

Causes of Fatigue

Most serious and many minor illnesses cause fatigue. However, most of these disorders have other more prominent symptoms (for example, pain, cough, fever, or jaundice) that are likely to bring the person to the doctor. This discussion focuses on disorders in which fatigue is the first or most severe symptom.

Common causes

There is no firm dividing line between causes based on duration of fatigue. However, doctors find that certain causes tend to be more common depending on how long people have had fatigue before they seek medical care.

Recent fatigue (lasting less than 1 month) has many causes, but the most common are the following:

For prolonged fatigue (lasting 1 to 6 months), the most common causes are the following:

For chronic fatigue (lasting longer than 6 months), the most common causes are the following:

Chronic fatigue syndrome is a disorder of unknown cause that results in fatigue and certain other symptoms. Not everyone who has fatigue for no apparent reason has chronic fatigue syndrome. People with COVID-19 COVID-19 COVID-19 is an acute respiratory illness that can be severe and is caused by the coronavirus named SARS-CoV-2. Symptoms of COVID-19 vary significantly. Two types of tests can be used to diagnose... read more may have symptoms that last for weeks or even months, which is known as "long COVID" or "long-haul COVID" and resembles chronic fatigue syndrome.

Less common causes

Stopping cocaine can cause severe fatigue. Less common causes of prolonged or chronic fatigue include adrenal gland underactivity and pituitary gland underactivity.

Evaluation of Fatigue

Fatigue can be highly subjective. People vary in what they consider to be fatigue and how they describe it. There are also few ways to objectively confirm fatigue or tell how severe it is. Doctors usually start an evaluation by trying to distinguish true fatigue from other symptoms that people may refer to as fatigue.

Warning signs

In people with fatigue, certain symptoms and characteristics are cause for concern. They include

When to see a doctor

All people feel fatigue occasionally, and not every case of fatigue requires evaluation by a doctor, particularly those that accompany an acute illness (such as an acute infection) or that go away after a week or so. However, fatigue that seems to last longer or has no obvious explanation should be evaluated.

Older adults with a new or different headache or loss of vision and people who have serious accompanying symptoms should see a doctor immediately. Even if they have no other symptoms, older adults with fatigue should see their doctor as soon as possible. Other people who have other warning signs should see a doctor in a few days. People who have no warning signs should call their doctor. The doctor can decide how quickly they need to be seen. Typically a delay of a week or so is not harmful.

What the doctor does

Doctors first ask questions about the person's symptoms and medical history. Doctors then do a physical examination. What they find during the history and physical examination often suggests a cause of the fatigue and the tests that may need to be done (see table ).

Doctors ask the person

  • To describe what is meant by fatigue as precisely as possible

  • How long fatigue has lasted

  • When fatigue occurs in relation to rest and activity

  • What other symptoms occur (such as fever, night sweats, or shortness of breath)

  • What measures relieve or worsen fatigue

  • How fatigue affects the person's work and social activities

Doctors then do a physical examination. Because many disorders can cause fatigue, the physical examination is very thorough, particularly in people with chronic fatigue. In particular, doctors also do a neurologic examination to evaluate the person's muscle strength and tone, reflexes, gait, mood, and mental status. The history and physical examination are more likely to reveal the cause of fatigue of more recent onset. A cause is also more likely to be found when fatigue is one of many symptoms than when fatigue is the only symptom. Fatigue that worsens with activity and lessens with rest suggests a physical disorder.



The need for tests depends on what doctors find during the history and physical examination. For example, doctors test for human immunodeficiency virus infection Human Immunodeficiency Virus (HIV) Infection (HIV) and tuberculosis Tuberculosis (TB) Tuberculosis is a chronic contagious infection caused by the airborne bacteria Mycobacterium tuberculosis. It usually affects the lungs, but almost any organ can be involved. Tuberculosis... read more Tuberculosis (TB) if people have risk factors. Testing for other infections or cancer is usually done only when people's findings suggest these causes. In general, people who have had fatigue for a long time and those who have warning signs are more likely to require testing.

If people do not have any other findings besides fatigue, many doctors do a few common blood tests. For example, they may do a complete blood count, blood tests to measure liver, thyroid gland, and kidney function, and a blood test called the erythrocyte sedimentation rate that suggests the presence of inflammation. However, such blood testing often does not reveal the cause.

Treatment of Fatigue

Treatment is directed at the cause of the fatigue. People with chronic fatigue syndrome Chronic Fatigue Syndrome Chronic fatigue syndrome, also called myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), refers to long-standing severe and disabling fatigue without a proven physical or psychological... read more or fatigue with no clear cause may be helped with physical therapy that includes increasing degrees of exercise and with psychologic support (for example, cognitive-behavioral therapy). Focusing on improving sleep and improving pain may also be helpful.

Essentials for Older People: Fatigue

Although it is normal for people to slow down as they age, fatigue is not normal. Fatigue is more often the first symptom of a disorder in older people. For example, the first symptom of pneumonia Overview of Pneumonia Pneumonia is an infection of the small air sacs of the lungs (alveoli) and the tissues around them. Pneumonia is one of the most common causes of death worldwide. Often, pneumonia is the final... read more Overview of Pneumonia in an older woman may be fatigue rather than any pulmonary symptoms (such as cough or difficulty breathing) or fever. In older people, the first symptom of other disorders, such as giant cell arteritis Giant Cell Arteritis Giant cell arteritis is chronic inflammation of large and medium arteries of the head, neck, and upper body. Typically affected are the temporal arteries, which run through the temples and provide... read more , may also be fatigue. Because serious illness may become apparent soon after sudden fatigue in older people, it is important to determine the cause as quickly as possible.

Key Points

  • Fatigue is a common symptom.

  • Fatigue is especially concerning if accompanied by certain warning symptoms, such as persistent, unintentional weight loss or chronic fever or night sweats.

  • If a doctor uncovers no findings suggesting a cause of fatigue, tests are often not helpful in identifying the cause.

  • Successful treatment of chronic fatigue may take work and persistence.

  • Fatigue in older people is not a normal part of aging.

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