Skip to Content



Michael R. Wasserman

, MD, Los Angeles Jewish Home

Last full review/revision Mar 2021| Content last modified Mar 2021

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 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.

  • Weakness: Weakness is lack of muscle strength that makes it difficult for people to move the affected muscles. Weakness is typically a symptom of a nervous system or muscle disorder. Disorders such as myasthenia gravis and Eaton-Lambert syndrome can cause weakness that worsens with activity, which may be confused with fatigue.
  • Shortness of breath: People, such as those with certain heart and lung disorders, become short of breath with activity but do not feel fatigued at rest.
  • Drowsiness: Excessive sleepiness is a symptom of sleep deprivation (for example, caused by lifestyle or by disorders such as allergic rhinitis, gastroesophageal reflux, painful musculoskeletal disorders, sleep apnea, and severe long-lasting disorders). Yawning and lapsing into sleep during daytime hours are common. However, many people with fatigue have disturbed sleep, so symptoms of sleep deprivation and fatigue can overlap.

Warning signs

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

  • Persistent, unintentional weight loss
  • Chronic fever or night sweats
  • Swollen lymph nodes throughout the body
  • Muscle weakness and/or pain
  • Serious accompanying symptoms (for example, coughing up or vomiting blood, bloody or black stools, shortness of breath, swelling in the abdomen, confusion, or suicidal thoughts)
  • Involvement of more than one organ system (for example, rash plus joint pain and stiffness)
  • Headache or loss of vision, particularly with muscle pains, in an older adult

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 Some Common Causes and Features of Prolonged or Chronic Fatigue).

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

Women are asked about their menstrual history. All people are asked about diet, anxiety, depression, and alcohol and drug use (including use of over-the-counter and recreational drugs).

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.

Some Common Causes and Features of Prolonged or Chronic Fatigue


Common Features*


Blood disorders or cancers


Decreased exercise tolerance with shortness of breath during activity greater than expected for the activity

Sometimes paleness


Cancers (such as digestive tract cancer, lung cancer, leukemia, lymphoma, multiple myeloma)

Widespread lymph node swelling, weight loss, and night sweats

With multiple myeloma, low back or other bone pain, often severe at night



Chronic hepatitis

Sometimes jaundice, loss of appetite, and fluid in the abdomen

Blood tests to determine how well the liver is functioning and to identify the hepatitis virus

Sometimes liver biopsy



Sometimes cough and respiratory symptoms

Sometimes loss of taste and smell

Blood test for SARS-CoV-2

Cytomegalovirus infection

Enlarged spleen and lymph nodes, fever, and night sweats

Sometimes blood tests for antibodies to cytomegalovirus

Heart valve infection (endocarditis)

Fever and night sweats

Sometimes joint pains

Usually in people who have heart murmurs or inject drugs intravenously

Cultures of blood samples and echocardiography


Fever, night sweats, and frequent infections

Sometimes difficulty breathing, cough, diarrhea, and/or rash

HIV blood test


Recent sore throat and lymph node swelling

Mononucleosis blood test

Other infections (for example, fungal pneumonias such as histoplasmosis, parasitic infections, or tuberculosis)

Fever, night sweats, and weight loss

Sometimes cough, difficulty breathing, and coughing up blood

Tests based on which type of infection seems likely

Other disorders and causes

Chronic kidney disease

Shortness of breath, difficulty breathing when lying down that is relieved when upright (orthopnea), and/or swelling

Blood tests of kidney function

Connective tissue disorder (for example, rheumatoid arthritis or systemic lupus erythematosus [lupus])

Fever, night sweats, weight loss, joint pain, rash, and/or other organ involvement (for example, effects on the heart or lungs)

Blood tests for abnormal antibodies


A history of lack of exercise or being bedbound or hospitalized

Only a doctor's examination

Diabetes (sometimes, other symptoms often more prominent)

Excessive thirst, excessive urination, increased appetite, and unexplained weight gain or loss

Measurement of sugar (glucose) level in the blood after the person fasts overnight and sometimes glucose tolerance testing

Drugs: Antidepressants, older antihistamines, antihypertensives, diuretics that cause low potassium levels in the blood, muscle relaxants, recreational drugs, and sedatives

History of taking a drug known to cause fatigue

Only a doctor's examination

Giant cell temporal arteritis

Headache, pain in the jaw when chewing, pain when combing hair, loss of vision, and/or muscle pains in a person over 50

ESR and temporal artery biopsy

Mental health disorders: Anxiety, depression, drug addiction, panic disorder, or somatization disorder (physical symptoms caused mainly by anxiety)

Anxiety, sadness, loss of appetite, and unexplained sleep disturbance

With somatization disorder, an excessive preoccupation with physical symptoms

Only a doctor's examination

Multiple sclerosis

Fatigue worse with exposure to heat

Past symptoms of nervous system malfunction (such as numbness, loss of coordination, and weakness), particularly if people had more than one episode of symptoms

Brain and/or spinal cord MRI


Cessation of menstrual periods, breast tenderness, morning sickness, and abdominal swelling

Pregnancy test

Sleep disorders

Excessive daytime sleepiness, frequent awakenings, breathing interruptions during sleep, difficulty falling asleep, unrefreshing sleep

Sleep laboratory testing (polysomnography)

Underactive thyroid gland (hypothyroidism)

Inability to tolerate cold, weight gain, constipation, and coarse skin

Blood tests of thyroid function


Weight loss

Sometimes loss of appetite, foul-smelling stool, abdominal pain, or a combination

A doctor's examination

Sometimes other tests

Disorders of unknown cause

Chronic fatigue syndrome (now sometimes called myalgic encephalomyelitis or systemic exertion intolerance)

Sore throat, sleep that is not refreshing, difficulty with concentration or short-term memory, muscle aches, joint pain, headaches, and/or tender lymph nodes in the neck or under the arms

Only a doctor's examination


Long-standing and widespread muscle and bone pain in areas outside the joints, trigger points, lower abdominal pain, gas, bloating, constipation or diarrhea, migraines, and anxiety

Only a doctor's examination

* Features include symptoms and the results of the doctor's examination. Features mentioned are typical but not always present.

CBC = complete blood count; ESR = erythrocyte sedimentation rate; HIV = human immunodeficiency virus; MRI = magnetic resonance imaging.


The need for tests depends on what doctors find during the history and physical examination. For example, doctors test for human immunodeficiency virus infection (HIV) and tuberculosis 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 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 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, 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 about Fatigue

  • Fatigue is a common symptom.
  • Fatigue caused primarily by a physical disorder increases with activity and lessens with rest.
  • 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.

Copyright © 2022 Merck & Co., Inc., known as MSD outside of the US, Kenilworth, New Jersey, USA. All rights reserved. Merck Manual Disclaimer