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Fainting (syncope) is a sudden loss of consciousness from a lack of blood flow to the brain. People who faint usually wake up quickly after collapsing.

Management for fainting is simple: Let the patient recover while lying flat. Equally important is treating the cause of the fainting, which isn't always easy to identify.

This article explains the symptoms and many possible reasons why someone may faint. It also details treatment options and how fainting may be prevented.

Causes of Fainting
Verywell / Joshua Seong

Fainting Overview

People who are prone to faint commonly begin to do so at about age 13. The person will feel flushed (warm or hot are also common feelings), followed by sudden weakness and loss of consciousness.

They'll go limp and often break out in a cold sweat. People who are standing when they faint will "pass out" and collapse to the ground.

Stimulation of the vagus nerve, which can cause the heart to slow and blood pressure to drop drastically, is one cause of fainting spells.

Once a person loses consciousness, the person's heart begins to speed up to counteract the low blood pressure.

Before Fainting

Before fainting, someone may exhibit or feel all or some of these signs and symptoms:

In cases of fainting caused by stimulation of the vagus nerve, a person may have cramps or an urge to have a bowel movement right before they pass out.

How Fainting Looks

In cartoons, fainting is often depicted with someone falling over, stiff as a board, and landing either flat on their face or back. In movies, the classic melodramatic faint comes with a gasp, a hand to the forehead, and a dramatic fall.

In real life, fainting ranges from subtle to violent. As blood flow to the brain slows, the brain stops sending signals to the muscle cells. The muscles lose their tone and the body collapses where gravity pulls it.

Occasionally, that sudden exit of blood from the brain leads to a small nervous impulse—kind of like static through a phone line. It may result in a tremble or shake. Sometimes it looks like a shudder; sometimes it looks like a seizure (albeit very short).

If you've ever felt an involuntary jerk in your arms or legs just as you fall asleep, it's called a myoclonic contraction. And it's exactly the same type of twitch some fainters exhibit.

While a myoclonic contraction is not a seizure, true seizures can also cause a sudden loss of consciousness. However, the symptoms of a seizure are different, as is the treatment of seizures.

After Fainting

Once the person goes from vertical to horizontal, blood starts flowing back to the brain and the person begins to wake up. It can be quick or it can take a while; everybody is different.

Similarly, the physical response can vary, too. People may:

  • Experience a rapid pulse or "racing heart"
  • Lose control of their bladder or bowels
  • Regain their color
  • Stop sweating

Causes of Fainting

Most fainting is triggered by the vagus nerve. It connects the digestive system to the brain, and its job is to manage blood flow to the stomach.

When food enters the system, the vagus nerve directs blood to the stomach and intestines, pulling it from other body tissues, including the brain.

Unfortunately, the vagus nerve can get a little too excited and pull too much blood from the brain. Some things make it work harder, such as bearing down to have a bowel movement or vomiting.

Medical conditions that drop blood pressure amplify the effects of the vagus nerve—even extreme pain from menstrual cramps.


Too little water in the bloodstream lowers blood pressure, and stimulating the vagus nerve when the system is already low can lead to dizziness and fainting.

There are many causes of dehydration, including vomiting or diarrhea, heat exhaustion, and burns. Vomiting and diarrhea, specifically, stimulate the vagus nerve.


Not all losses of consciousness are related to the vagus nerve. Shock is a condition characterized by low blood pressure that often leads to a loss of consciousness.

In general, people are more aware of the effects of high blood pressure, which is good. But very low blood pressure is much more immediately dangerous.

Shock is a life-threatening emergency that usually comes from bleeding, but can also come from severe allergy (anaphylaxis) or severe infection. People with shock will most likely become confused, then lose consciousness as their condition gets worse.

Drugs or Alcohol

Plenty of people lose consciousness due to alcohol use, and we don't call it fainting (although passing out seems appropriate).

Besides its obvious sedation effect, alcohol makes people urinate, which will eventually lead to dehydration. It also dilates blood vessels, which decreases blood pressure.

Like shock, losing consciousness due to alcohol is not technically considered fainting, but it may or may not be cause for concern. It is possible to die from alcohol poisoning, and passing out is a sign of serious intoxication.

Other drugs—legal as well as illegal—can knock you out for a variety of reasons:

  • Any drug meant to control high blood pressure acts in some way to lower blood pressure—and too much of these medications may cause low blood pressure.
  • Diuretics make you urinate and can lead to dehydration.
  • Heart drugs often lower blood pressure.
  • Nitrates quickly lower blood pressure.
  • Opiates slow breathing while lowering blood pressure.
  • Stimulants dry you out and raise your temperature.

Heart Rate

Your heart is the pump that forces blood through your veins and arteries. It takes a certain amount of pressure in the bloodstream to keep it flowing. A correctly functioning heart is essential to maintaining adequate blood pressure.

If the heart beats too fast or too slow, it can't keep the blood pressure up as high as it needs to be. Blood drains from the brain and leads to fainting. During a heart attack, the heart muscle can become too weak to maintain blood pressure.

To decide if the heart may be the culprit, take a pulse. If it's too fast (more than 150 beats per minute) or too slow (less than 50 beats per minute), it's fair to suspect that the heart caused the fainting spell.

If the person complains of chest pain or other symptoms of a heart attack, assume the heart is too weak to keep blood in the head.

Less Common Causes

Some people pass out when they see blood. Anxiety, panic disorder, and stress can stimulate the vagus nerve and lead to a loss of consciousness.

The vagus nerve stimulates the parasympathetic nervous system, which slows down the pulse rate and lowers blood pressure.  Some people are hypersensitive to the vagus nerve, and stimulation can result in a loss of consciousness.

If you see someone pass out, make sure the person is breathing. If not, call 911 and begin CPR.

How to Treat Fainting

All by itself, fainting is not life-threatening. However, sudden cardiac arrest looks a lot like fainting and requires immediate treatment.

Once someone faints, get the patient comfortably lying flat. You can elevate the legs to help blood flow return to the brain.

After this, treatment depends on the cause of fainting. If this is the first time the person has ever fainted—or if you don't know—call 911.

There are some dangerous conditions that can cause fainting. And they should be evaluated by medical professionals to determine how to proceed.

If the person has a history of fainting, watch their breathing and give them a couple of minutes to wake up. If the person doesn't wake up within three minutes of lying flat, call 911.


Sometimes, there's absolutely nothing you can do to stop from fainting. But if you feel it coming on, there are a few things that may help.

If you feel suddenly flushed, hot, or nauseated, or break out in a cold sweat, don't stand up. Lie down until that "woozy" feeling passes.

If the feeling doesn't pass in a few minutes, or you begin to experience chest pain or shortness of breath, call 911.


It never looks as poetic as it can in the movies, but fainting is a sudden loss of consciousness from a lack of blood flow to the brain. It can be triggered by many things, including dehydration, shock, too much alcohol, and even anxiety. While a fainting spell usually doesn't last for long, the trick is learning to distinguish it from a life-threatening event, like a heart attack. This can happen when the heart muscle becomes too weak to maintain blood pressure.

A Word From Verywell

Passing out can be a scary event. But understanding fainting is half the battle in preventing it. People who have experienced multiple fainting spells should see a healthcare provider to learn the cause. If nothing else, they can learn the warning signs and symptoms of fainting and take steps to avoid it.

Frequently Asked Questions

  • What should you do after fainting?

    Call your healthcare provider to let them know you had a fainting episode. Tests like an electrocardiogram, echocardiogram, and a Holter monitor can check for heart problems. A table tilt test can monitor your vitals while you're upright and lying flat.

  • Does locking your knees when standing cause you to faint?

    Yes, it can. When you stand too long in one place with your knees locked straight, it can cause your blood to pool in your leg veins, which may cause you to faint. To prevent this from happening, make sure to bend your knees or move around a little to improve your circulation.

  • How can you prevent fainting when getting your blood drawn?

    Find ways to distract yourself. Look away when the test is being done, talk to the person doing the blood draw, or try some meditation and deep breathing exercises. Also, let the phlebotomist know if you're prone to fainting. They may be able to let you lie down during the test.

4 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  2. Bergfeldt L. Differential diagnosis of cardiogenic syncope and seizure disorders. Heart. 2003;89(3):353-8. doi:10.1136/heart.89.3.353

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Additional Reading

By Rod Brouhard, EMT-P
Rod Brouhard is an emergency medical technician paramedic (EMT-P), journalist, educator, and advocate for emergency medical service providers and patients.