11 Causes of Sudden Fatigue or Exhaustion

As a symptom, fatigue is a tough one to quantify. “Fatigue is defined as tiredness, exhaustion, or lack of energy precipitated by exertion or stress,” says Andrew Patane, MD, an internal medicine doctor at NYU Langone Health in New York City. “It’s a subjective symptom and, unfortunately, we don’t have objective [ways to measure it with an exam].”

If you’re overwhelmed by exhaustion, it could be a sign of an issue that needs addressing. Lifestyle factors, medications, and health conditions can all cause extreme fatigue. Here are 11 potential underlying causes.

1. Illness or Infection

Sudden fatigue could be a sign of a viral or bacterial infection.

• Flu
• COVID-19
• Lyme disease
• Mononucleosis (mono)

Fever is common with all of these illnesses, but other signs and symptoms may vary.

• Urinary tract infections (including bladder and kidney infections)
• Bacterial pneumonia
• Dental abscesses
• Sinus infections

In addition to fever, most bacterial infections cause chills and pain or discomfort in the affected area.

2. A Vitamin or Nutrient Deficiency

These side effects are usually temporary. But if ketosis is causing your sudden energy crash, you may want to add more fiber-rich, complex carbohydrates to your diet. Those include fruits, vegetables, and whole grains like oatmeal and brown rice.

But keep in mind that those are bare minimums. A calorie-counting app, or the calculator from the U.S. Department of Agriculture (USDA), can provide an estimate of your ideal daily calorie requirements.

3. Certain Lifestyle Habits

This is one of the first areas Dr. Patane says he checks on when he sees patients for fatigue. He may ask them if they are:

• Drinking more alcohol than usual
• Doing extreme workouts, such as training for an endurance race
• Undereating
• Working more than usual
• Doing night-shift work
• Having a work-life imbalance
• Practicing poor sleep habits

While some of these might sound obvious, it sometimes takes the help of a professional to point them out and guide you in making the right changes.

4. A New Medication

Medications are meant to make you feel better, but starting a new prescription drug often causes fatigue as a side effect.

Some medications that can cause fatigue, according to Patane, include:

• Antidepressants
• Antihistamines
• Beta-blockers for high blood pressure
• Benzodiazepines for anxiety

If you’ve recently started a new medication, talk to your doctor. They may be able to change your prescription or alter the dose to treat you while minimizing these side effects.

5. Stress, Anxiety, or Depression

Mental health issues are known for dragging down your energy. “These can all present with fatigue,” Patane says.

6. Heart Attack or Stroke

If you have sudden exhaustion along with any other symptoms of a heart attack or stroke, call 911 immediately. In either case, your likelihood of recovery is better the sooner you get treatment.

7. Autoimmune Diseases

Fatigue is the most commonly reported symptom across these diseases, and sometimes the sudden bouts of extreme fatigue can be profound and debilitating. The reason? High levels of inflammation contribute to the kind of tiredness that can make it hard to carry out everyday tasks.

These conditions can be tricky to diagnose, in part because symptoms like fatigue can be so nonspecific. They do tend to run in families, though, so be sure to tell your doctor about your family health history.

8. Anemia

The condition can cause sudden lethargy and is more common in menstruating women, endurance athletes, and people with kidney or digestive diseases.

Your doctor can diagnose you based on a physical exam and blood tests. They may recommend iron supplements or diet changes to help replenish your iron stores and energy.

9. Postural Orthostatic Tachycardia Syndrome (POTS)

Some people with POTS describe it as having sudden fatigue attacks. Others say it’s like having waves of sudden extreme fatigue and nausea, or extreme fatigue “out of nowhere.” It can appear differently depending on the person, but other symptoms might include:

• A feeling of lightheadedness or dizziness when standing, or fainting
• Nausea and vomiting
• Brain fog
• Muscle cramps or muscle pain
• Headaches
• A shaky feeling
• Sweating a lot

POTS tends to run in families, and there’s a link between POTS and having highly mobile joints (Ehlers-Danlos syndrome), according to Johns Hopkins Medicine. If you think you might have it, see your doctor, who can run a test and make a diagnosis.

10. Chronic Fatigue Syndrome

The hallmark symptom of chronic fatigue syndrome (CFS), also known as myalgic encephalomyelitis (ME), is profound fatigue that lasts at least six months, doesn’t go away with rest, and can’t be explained by an underlying medical condition. While it’s a long-term condition, it may come on suddenly.

Other symptoms of CFS include:

• Problems with memory or thinking skills
• Dizziness
• Muscle or joint pain
• Trouble sleeping or sleep that isn’t refreshing

11. Cancer

When there’s something wrong, it can be hard not to think about the worst-case scenario. So it’s important to get checked out by your doctor before making any assumptions. But when you’re telling your doctor about your symptoms, they may wonder if there’s any reason to suspect cancer.

When to Call Your Doctor

If fatigue keeps you from taking part in work or school, or social or personal activities, then you should see your doctor, Patane says.

“When in doubt, it never hurts to speak to your doctor about this. It can be a simple evaluation, and we may be able to say there's nothing seriously wrong and suggest lifestyle modifications. One of the best ways to [address] fatigue is with good sleep hygiene and by slowly increasing daily exercise.”