What causes lethargy? 45 possible conditions

Lethargy is a symptom that causes you to feel sleepy or fatigued and sluggish. This sluggishness may be in terms of movements or in thinking. You’re described as lethargic if you have these symptoms.

Lethargy can be related to an underlying physical or mental condition.

Lethargy can cause some or all of the following symptoms:

• changes in mood
• decreased alertness/ability to think
• fatigue
• low energy
• sluggishness

People with lethargy may act as if they are in a daze. They may move more slowly than they usually do.

Many kinds of acute illnesses can make you feel lethargic. This includes the flu or a stomach virus. Other physical medical conditions can also cause lethargy. These include:

• carbon monoxide poisoning
• dehydration
• hyperthyroidism
• hypothyroidism
• hydrocephalus (brain swelling)
• kidney failure
• Lyme disease
• meningitis
• pituitary diseases, such as pituitary cancer
• poor nutrition
• sleep apnea
• stroke
• traumatic brain injury
• fever

Lethargy can be the result of mental health-related medical conditions. These include:

• major depression
• postpartum depression
• premenstrual syndrome (PMS)

Lethargy can also be a side effect of taking certain medications. Taking narcotic medications can cause lethargy. You shouldn’t exceed your recommended dosage for this reason.

Lethargy can be a symptom that requires emergency medical attention. This is especially true if lethargy symptoms come on suddenly. Additional symptoms that may require emergency medical attention:

• chest pain
• unresponsiveness or minimally responsiveness
• inability to move the limbs on one side of the body
• disorientation (e.g., not knowing date, name, or location)
• fast heartbeat
• facial drooping on one side
• loss of consciousness
• rectal bleeding
• severe headache
• shortness of breath
• vomiting blood

Any noticeable, marked changes in behavior accompanied by lethargy are often cause for concern. Seek immediate medical attention if you experience thoughts of harming yourself as well as lethargy.

Babies or young children can also experience lethargy. Symptoms in babies that may need immediate medical attention include:

• difficult to rouse
• fever greater than 102 degrees Fahrenheit
• dehydration symptoms, such as crying without tears, dry mouth, or few wet diapers
• sudden rash
• vomiting forcefully, especially for more than 12 hours

You may also want to make an appointment at your doctor’s office if you have any of these symptoms as well as lethargy:

• aches and pains that don’t go away with treatment
• difficulty sleeping
• difficulty tolerating hot or cold temperatures
• eye irritation
• fatigue that lasts longer than two weeks
• feeling sad or empty frequently
• irritability
• swollen neck glands
• unexplained weight gain or loss

Your doctor will often take a full medical history and then do a physical exam to figure out why you are experiencing lethargy. Obtaining a medical history includes discussing any medical conditions you may have previously experienced. The medical examination may include listening to your heart and lungs, testing for bowel sounds, and evaluating your mental awareness.

Diagnostic testing typically depends on what the doctor suspects may be an underlying cause. For example, if a thyroid disorder is suspected, your doctor may order blood tests to determine if your thyroid hormones are high or low.

Your doctor may order imaging studies if they suspect a head injury, stroke, meningitis, or other brain-related disorder is the cause. The imaging studies could include a computed tomography (CT) or magnetic resonance imaging (MRI) scan to determine if there are any abnormalities in the brain.

Treatment for lethargy depends upon its underlying cause. Your doctor may prescribe medications to resolve an infection or to increase thyroid hormones. They may prescribe antidepressants if depression or another mental health-related disorder is the cause.

Remember that you can practice healthy habits at home to reduce fatigue-related symptoms if you experience lethargy. Examples include:

• drinking plenty of fluids
• eating a healthy diet
• getting plenty of sleep
• reducing stress levels

See a doctor if these healthy habits don’t help your symptoms go away.