“Lethargy” is a term that no longer sees widespread use in medical settings. It refers to a decrease in consciousness, but many people use it interchangeably to refer to fatigue, drowsiness or sleepiness. Because it involves a decrease in consciousness, it indicates a disruption in brain activity.


What is lethargy?

Lethargy is a symptom that involves an unusual decrease in consciousness. It’s different from just being drowsy or sleepy. Decreases in your level of consciousness can involve a change in your mental state. That means you may be confused or have trouble remembering things, too.

Lethargy comes from two Ancient Greek words: “Lethe," meaning “forgetful,” and “argos,” meaning “idle.” As those two terms describe, someone who’s lethargic moves slowly or is difficult to rouse and has trouble thinking, concentrating or remembering.

Fatigue vs. lethargy — what’s the difference?

Fatigue is feeling physically exhausted or drained but without any disruption or impairment of your mental abilities. While people often use the term “lethargic” to describe someone who’s tired, fatigued or drowsy, they’re not the same. Lethargy indicates something is affecting your brain, your level of consciousness and other mental functions. But it’s also hard to tell the difference — even for healthcare providers — between lethargy and fatigue or being very tired. Context and additional information can be key in determining if someone has lethargy as a symptom or if they’re really tired.

Lethargy and its place in medical terminology

In the medical world, “lethargy” isn’t a term that’s as commonly used as in years past. There are a few reasons for this:

  • It’s not specific. Lethargy doesn’t specify what brain processes are experiencing disruption. It can involve energy level changes, memory problems, reduced thinking ability, a lack of awareness or responsiveness to the world around you, trouble concentrating or other changes.
  • It’s not precise. Consciousness ranges from being in the deepest coma to being completely awake, alert and oriented. Lethargy falls somewhere in between, but there’s no way to pinpoint its exact location in that range.
  • It’s confusing. As noted earlier, people commonly use lethargy as a catch-all term. Using it in a medical context can lead to miscommunication if anyone interprets it differently than it was originally intended.


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Possible Causes

What can cause lethargy?

Lethargy can be a sign of many conditions. But what they all have in common is that they involve disruptions in brain function.

Conditions that can cause lethargy include (but aren’t limited to):

Care and Treatment

How is lethargy treated?

Treating lethargy almost always involves treating the condition causing it. Because so many conditions can cause or contribute to lethargy, the possible treatments can vary widely.

What can I do at home to treat lethargy?

Lethargy is a possible symptom of a life-threatening medical emergency. You shouldn’t try to treat it on your own.

What are the possible complications or risks of not treating lethargy?

Lethargy can signal severe or life-threatening medical conditions. Many of these conditions can cause brain damage or even death without treatment.


Can lethargy be prevented?

Lethargy happens unpredictably. The conditions that can cause it generally aren’t preventable. But there are some things you can do to reduce the risk of developing these conditions.

  • Manage your chronic conditions. Following your healthcare provider’s guidance on managing chronic conditions like diabetes and epilepsy can lower your odds of developing a condition that can cause lethargy.
  • Wear safety equipment. Head injuries, especially concussions and traumatic brain injuries, can cause or contribute to lethargy. Helmets, safety restraints, seat belts and other protective gear are essential for reducing your risk.
  • Eat a balanced diet. A balanced diet can reduce the risk of developing electrolyte imbalances and vitamin or mineral deficiencies that could cause lethargy. Your diet also affects your circulatory health, which can help avoid conditions like stroke.
  • Stay physically active and reach and maintain a weight that’s healthy for you. Your weight and activity level can prevent or delay conditions that affect your brain, especially conditions that could cause or contribute to lethargy.
  • Avoid substance and nonmedical drug use, and only use alcohol in moderation. Alcohol and substance use disorders can cause or contribute to lethargy. You should always take prescription medications as directed, as this reduces the risk of developing problems or complications.
  • Get adequate sleep. Most adults require seven to eight hours of sleep every night. Insufficient sleep can worsen your symptoms.

When To Call the Doctor

When should this symptom be treated by a doctor or healthcare provider?

Lethargy can be hard to distinguish from drowsiness, even for trained, experienced healthcare providers. An important thing to be aware of when considering if someone has lethargy is the context.

Context clues to consider include:

  • Recent events or circumstances. This includes recent injuries, medical procedures, etc.
  • How suddenly they became lethargic. Suddenly becoming drowsy and having trouble thinking without any obvious reason is a red flag.
  • Behavior patterns. If the sudden change in activity level, alertness or behavior is unusual or seems out of character, that’s a reason to seek medical attention.
  • Changes in the moment. If someone seems lethargic but gradually becomes more alert and aware, it’s likely not lethargy. If they stay lethargic or get worse over several minutes, they need medical attention.
  • If they have other symptoms. Some symptoms, like those of a stroke, automatically mean that someone needs emergency care. If these symptoms appear, you should call 911 or your local emergency services number immediately because some conditions that cause lethargy are time-sensitive. Every second counts.

In general, it’s best to err on the side of caution when considering whether or not to get medical care for suspected lethargy.

A note from Cleveland Clinic

It can be hard to tell the difference between fatigue and lethargy. The key difference is that lethargy involves a decrease in consciousness. That means you’re less awake, have diminished awareness or are disoriented. Lethargy happens when there’s a disruption in your brain, and it can be a sign of serious or even life-threatening conditions. If you suspect someone has lethargy, it’s best to err on the side of caution. Get medical attention. Many conditions that can cause lethargy are time-sensitive, so acting out of caution is the wisest course of action.

Medically Reviewed

Last reviewed on 09/11/2023.

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