Muscle weakness is a lack of strength in the muscles. They may not contract or move as easily as before.

Some chronic medical conditions can cause the muscles to wear out more quickly or cause a person to feel fatigued. In other cases, an infection may cause the muscles to falter.

If a person has a sudden, severe onset of muscle weakness, they should talk to a doctor.

Examples of conditions that cause muscle weakness include:

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A person with Addison's disease may experience chronic fatigue or loss of appetite.

This occurs when a person's adrenal glands do not produce enough of the hormones cortisol and aldosterone.

In addition to muscle weakness, other common symptoms of Addison's disease include:

  • chronic fatigue
  • weight loss
  • loss of appetite
  • stomach pain

Anemia occurs when a person's hemoglobin levels are low, often due to an iron deficiency. Other symptoms of anemia include:

  • dizziness
  • shortness of breath
  • headaches
  • cold hands and feet
  • an irregular heartbeat

This diagnosis refers to unexplained fatigue, or fatigue that a doctor cannot relate to a medical condition. Another name for it is myalgic encephalomyelitis.

People with chronic fatigue syndrome experience severe tiredness and sleep problems. Other symptoms include muscle weakness, pain, dizziness, and problems concentrating.

Electrolytes help ensure that the muscles, nerves, heart, and brain all function correctly. Having altered levels of electrolytes — such as calcium, potassium, sodium, and magnesium — can cause muscle weakness.

Examples of electrolyte disorders include hypokalemia or hyperkalemic periodic paralysis.

Risk factors for an electrolyte imbalance include:

Diabetes occurs when the body does not make enough insulin or does not use insulin appropriately. It can cause nerve damage that may result in muscle weakness.

Diabetes can also lead to a variety of other symptoms related to muscle weakness, including:

  • frailty
  • impaired mobility
  • fatigue

Fibromyalgia is a chronic condition that causes muscle pain and weakness in addition to other symptoms, such as:

  • constant fatigue
  • affected memory
  • mood changes

Hypothyroidism, or an underactive thyroid, can cause muscle weakness and cramping. These symptoms may get worse with exercise and physical activity.

Other symptoms include:

  • weight gain
  • feeling cold
  • dry skin and hair
  • fatigue
  • irregular or heavy menstrual periods
  • a slow heart rate
  • joint and muscle pain
  • depression or mood disorders
  • fertility problems

A doctor can often diagnose this and other thyroid conditions with a blood test.

Problems with kidney function can cause metabolic waste products, such as creatinine, to build up in the muscles. This can result in muscle twitching and weakness.

Sleep disorders, such as narcolepsy and insomnia, can result in daytime muscle weakness and fatigue.

A person who needs to stay in bed due to a medical condition may also experience muscle weakness. This results from not using the muscles as regularly as usual. A person may also be at risk of sleep problems.

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Influenza can cause muscle weakness, a fever, and a sore throat.

Some infectious diseases can cause muscle weakness. They include:

  • Influenza: The influenza (flu) virus can cause temporary muscle weakness as well as a fever, sore throat, cough, and fatigue.
  • Lyme disease: This inflammatory disease follows a bite from an infected tick. Symptoms can be acute or chronic and include a fever, rash, neck stiffness, numbness, muscle weakness, and fatigue.
  • Epstein-Barr virus: The Epstein-Barr virus can result in muscle weakness as well as unexplained fatigue, a skin rash, headaches, and appetite loss.
  • Syphilis: This sexually transmitted infection can cause muscle weakness, as well as headaches, fatigue, a sore throat, and weight loss.
  • Toxoplasmosis: Toxoplasmosis is a parasitic infection that causes headaches, fatigue, a low-grade fever, and seizures.
  • Meningitis: Meningitis is a serious infection that causes inflammation in the brain and spinal cord. In addition to muscle weakness, symptoms can include a fever, neck stiffness, nausea, vomiting, and increased sensitivity to light.
  • HIV: HIV can cause progressive muscle weakness in some individuals, especially in those who do not receive treatment.
  • Polio: Polio myositis can cause muscle weakness and sensitivity. Also, a person who has had polio can experience post-polio syndrome, which results in muscle weakness.
  • Rabies: Rabies results from contact with the saliva of an infected animal. Symptoms can include fatigue, headaches, agitation, confusion, and seizures, as well as muscle weakness and spasms.

Some conditions that affect the nervous system can cause muscle weakness. These conditions are often chronic and affect the way that a person's nerves transmit messages to their muscles.

Examples of neurological conditions that can cause muscle weakness include:

  • Cervical spondylosis: Age-related changes to the cushioning spinal disks in the neck can cause cervical spondylosis. This puts extra pressure on nerves, resulting in muscle weakness.
  • Guillain-Barré syndrome: This rare neurological disorder can cause mild-to-severe muscle weakness.
  • Botulism: This rare condition occurs due to exposure from botulinum toxin. It also causes progressive muscle weakness.
  • Lambert-Eaton myasthenic syndrome: This autoimmune disorder occurs when a person's immune system interferes with how the nerves and muscles communicate, resulting in muscle weakness.
  • Multiple sclerosis: Multiple sclerosis is an autoimmune disorder that occurs when the immune system attacks and damages the nerves.
  • Myasthenia gravis: This autoimmune disorder causes the immune system to attack a person's muscles, which can affect movement as well as breathing.
  • Amyotrophic lateral sclerosis: Known as ALS, this can cause progressive muscle weakness.
  • Spinal cord injuries: Injuries to the spinal cord can interrupt communication from the nerves to the muscles. The effects can depend upon the exact site of the injury.

Neurological conditions are often progressive, which means that they get worse over time.

Some of these conditions also go through stages of remission, when symptoms lessen or even disappear, before flaring up again.

Some people experience muscle weakness as a result of medicines that they take.

Anyone experiencing muscle weakness as a side effect should speak to a doctor before stopping their medication.

Examples of medications that can cause muscle weakness include:

  • amiodarone (Cordarone)
  • antithyroid medications, such as methimazole (Tapazole) or propylthiouracil
  • antiretroviral medicines, such as lamivudine (Epivir) or zidovudine (Retrovir)
  • chemotherapy medications
  • cimetidine (Tagamet)
  • corticosteroids
  • fibric acid derivatives, such as gemfibrozil (Lopid)
  • interferon
  • leuprolide acetate (Lupron)
  • nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen or naproxen
  • penicillin
  • statins
  • sulfonamide antibiotics

Some illicit drugs, such as cocaine, can also cause muscle weakness.

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Rare causes of muscle weakness include dermatomyositis, polymyositis, and rheumatoid arthritis.

If a person's muscle weakness is not due to any of the issues above, or if they have particular risk factors, a doctor may consider rare causes when making their diagnosis.

Rare causes of muscle weakness include:

  • Dermatomyositis: This is an inflammatory muscular disorder that can cause stiff, sore, and weakened muscles.
  • Polymyositis: This usually causes weakness in the muscles near the body's trunk, such as the hip, thigh, neck, and shoulder muscles.
  • Rheumatoid arthritis: This is a chronic, inflammatory autoimmune disorder that attacks the lining of the joints. Commonly affected areas include the hands and feet.
  • Sarcoidosis: This inflammatory condition usually affects the lung and lymph glands, causing irritated masses of tissue.
  • Secondary hyperparathyroidism: This condition often affects the lower extremities and causes bone and joint pain.
  • Becker muscular dystrophy: This genetic disorder usually affects males and younger people and results in rapid, progressive muscle weakness.
  • Systemic lupus erythematosus: Also known as lupus, this condition can affect various areas of the body, including the joints, brain, heart, and lungs. Muscle weakness is a common symptom of a lupus flare-up.

These conditions are not necessarily rare in the population, but muscle weakness is not always included among their common symptoms.

Multiple medical issues can cause muscle weakness, including neurological conditions, infections, and chronic illnesses.

To diagnose the underlying cause, a doctor will discuss a person's symptoms, medical history, and what makes the symptoms worse or better.

If a person experiences sudden, severe muscle weakness, they should seek immediate medical attention.