Muscle weakness may be due to a chronic condition or an infection. Conditions that can weaken muscles include Addison’s disease, anemia, diabetes, fibromyalgia, and more.

Muscle weakness is a lack of muscle strength, meaning the muscles may not contract or move as easily as they used to. If a person has a sudden, severe onset of muscle weakness, they should talk with a doctor.

Examples of conditions that cause muscle weakness include:

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Addison’s disease 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 include:

Anemia occurs when a person’s hemoglobin levels are low, often due to an iron deficiency.

Other symptoms of anemia include:

Also known as myalgic encephalomyelitis, chronic fatigue syndrome is a complex condition that can affect various systems and functions in the body.

People with chronic fatigue syndrome experience:

Other symptoms can include:

Electrolytes help ensure that the muscles, nerves, heart, and brain all function correctly. Examples of electrolytes include:

Having altered levels of electrolytes can cause muscle weakness.

Types of electrolyte disorders include hypokalemia and 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:

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

  • constant fatigue
  • memory issues
  • mood changes

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

Other symptoms include:

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 buildup can lead to muscle twitching and weakness.

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

Muscle weakness can result from a lack of use, such as when a person is on bed rest for an extended period due to a medical condition or spends time in a hospital.

A 2023 study found that, on average, critically ill people lose about 2% of skeletal muscle per day during the first week of admission in an intensive care unit (ICU).

Some infectious conditions can cause muscle weakness. They include:

  • Flu: The flu virus can cause the following symptoms:
  • Lyme disease: This inflammatory condition occurs after a bite from an infected tick. Symptoms can be acute or chronic and include:
  • Epstein-Barr virus: The Epstein-Barr virus can result in:
    • muscle weakness
    • fatigue
    • a rash
    • headaches
    • appetite loss
  • Syphilis: This sexually transmitted infection can cause:
    • muscle weakness
    • headaches
    • fatigue
    • a sore throat
    • weight loss
  • Toxoplasmosis: Toxoplasmosis is a parasitic infection that also causes:
    • headaches
    • fatigue
    • a low grade fever
    • seizures
  • Meningitis: Meningitis is a serious infection that leads to inflammation in the brain and spinal cord. In addition to muscle weakness, symptoms can include:
  • HIV: HIV can cause progressive muscle weakness in some individuals, especially those who do not receive treatment.
  • Polio: Polio myositis can cause muscle weakness and sensitivity. A person who has had polio can also experience post-polio syndrome, which results in muscle weakness.
  • Rabies: Rabies results from contact with the saliva of an animal with the infection. Symptoms can include:
  • COVID-19: The SARS-CoV-2 virus can cause this illness, which can lead to cold-like symptoms, including:
    • fever
    • chills
    • muscle aches or body pain
    • muscle weakness weeks after the initial diagnosis

Some conditions that affect the nervous system can cause muscle weakness. These conditions are often chronic and affect how 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, which puts extra pressure on nerves and results in muscle weakness.
  • Guillain-Barré syndrome: This rare neurological disorder can cause mild to severe muscle weakness.
  • Botulism: Botulism is a rare condition that occurs due to exposure to botulinum toxin. It 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 (MS): MS is an autoimmune disorder in which the immune system attacks and damages the nerves in the brain and spinal cord.
  • Amyotrophic lateral sclerosis (ALS): This condition leads to degeneration of the nerve cells in the brain and spinal cord, causing progressive muscle weakness.
  • Myasthenia gravis: This autoimmune disorder causes the immune system to attack a person’s muscles, which can affect movement and breathing.
  • 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 have stages of remission, when symptoms lessen or even disappear for a period.

Some people experience muscle weakness as a result of the medication they take.

Anyone experiencing muscle weakness as a side effect should speak with a doctor. They should not stop taking their medication without medical advice.

Examples of medications that can cause muscle weakness include:

Some illegal drugs, such as cocaine, can cause muscle weakness, as can an opioid overdose.

As people age, they begin to lose muscle mass and strength (sarcopenia). This tends to start when a person is around 40 years old.

The main symptom of sarcopenia is muscle weakness. Other symptoms can include:

Muscle strain, also called a pulled muscle, is an injury in which the muscle rips or tears. A person may injure a muscle due to:

  • overuse or improper use
  • playing certain sports
  • fatigue

Strains can occur in any muscle and may cause weakness. They can vary in severity. If the injury is not serious, icing the area and rest can help.

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 other 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 tissue masses.
  • 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, but muscle weakness is not always 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 ask about a person’s symptoms, medical history, and whether anything makes the symptoms worse or better.

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