Fainting Causes, Symptoms, Treatment, and Prevention

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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 (supine). Equally as important as immediate management is treating the cause of the fainting.

Causes of Fainting
Verywell / Joshua Seong

Fainting Symptoms

Folks who are prone to syncope commonly begin fainting at around 13 years old. The person will feel flush (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, or "pass out," will 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 patient's heart begins to speed up in order to fix the low blood pressure.

Before Fainting

Before fainting, a patient can exhibit or feel all or some of these signs and symptoms, depending on the cause of fainting:

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

How Fainting Looks

In cartoons, fainting is always done by falling over stiff as a board and landing either flat on your face or back. In movies, the classic melodramatic faint is with a gasp, a hand to the forehead and collapsing into the arms of the nearest male heartthrob.

In real life, fainting ranges from subtle to violent. As the brain stops getting enough blood flow to stay conscious, it stops sending out signals to the muscle cells. The muscles lose their tone and the body just collapses into whatever heap gravity pulls it to.

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

Ever felt an involuntary jerk in your arms or legs just as you fall asleep? That is called a myoclonic contraction, and it's exactly the same type of twitch some fainting patients 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 into the brain and they begin to wake up. It can be quick or it can take a while; everybody's different.

Some of the more common symptoms that can occur after fainting:

  • Sweating stops
  • Color begins to return
  • Rapid pulse or "racing heart"
  • Loss of bowel or bladder control (incontinence)


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 gut. 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—vomiting or diarrhea, heat exhaustion, burns, and more. Vomiting and diarrhea, specifically, also 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. As a society, we are very aware of the long-term consequences of high blood pressure, 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.

It can all happen very quickly, and although it's not fainting, per se, we can't really tell unless the patient wakes up. Taking a wait-and-see attitude may be dangerous.

Drugs or Alcohol

Plenty of people lose consciousness due to alcohol use, and we don't call it fainting (although passing out still seems appropriate). Besides its obvious sedation effect, alcohol makes you 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:

  • Nitrates quickly lower blood pressure.
  • Diuretics make you urinate and can lead to dehydration.
  • Stimulants dry you out and raise your temperature.
  • Opiates lower blood pressure and slow breathing.
  • Heart drugs often lower blood pressure.
  • 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.

Heart Beat

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), suspect that the heart caused the fainting spell. Also, if the patient is complaining 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

Do you pass out when you see blood? Anxiety, panic disorder, and stress can stimulate the vagus nerve in some people 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 loss of consciousness in these people.


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

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

Once someone faints, get the patient comfortably lying flat. You can elevate the legs to help blood flow return to the brain, but it is generally not necessary and there's some debate on whether it is effective.

Treatment after that depends on the cause of fainting. If this is the first time this person has ever fainted—or if you don't know—call 911. There are some dangerous conditions that can cause fainting and should be evaluated by medical professionals to determine how to proceed.

If the person has a history of fainting, watch the 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.

More important than immediate treatment is to treat the cause of the fainting. Often, the only way to identify the cause is to look at the patient 's chronic medical problems, if any, and recent activities or illnesses.


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

A Word From Verywell

Passing out is, undoubtedly, a scary event. Knowledge is half the battle in prevention. Patients of multiple fainting spells should definitely see a healthcare provider and determine the cause of the fainting (if any). Patients will often learn the warning signs and symptoms of fainting and can learn 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. You may need to have tests to check for any heart problems, including an electrocardiogram, echocardiogram, and a Holter monitor. They may also suggest a table tilt test, which monitors your vitals when you're upright and when lying flat.

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

    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 help improve circulation.

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

    If you know that you usually faint at the sight of blood, such as when you get 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, since they may be able to let you lie down while having the test done.

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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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