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Causes of Fainting

There Are Lots of Reasons Why We Pass Out


Updated May 20, 2014

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

Woman drinking water, side view
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Fainting (syncope) is a sudden loss of consciousness from a lack of blood flow to the brain. Victims usually wake up quickly after collapsing. Management for this condition is simple, usually requiring little more than letting the victim recover while lying flat (supine). More important than immediate management is treating the cause of the fainting. Often, the only way to identify the cause is to look at the victim's chronic medical problems, if any, and recent activities or illnesses.

Most fainting is triggered by the vagus nerve. It connects the digestive system to the brain, and it's 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. Conditions that drop blood pressure amplify the effects of the vagus nerve.

Folks who are prone to this condition (syncope) commonly begin fainting at around 13 years old and continue for the rest of their lives. Fainting usually follows a pattern. The victim 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. Victims who are standing when they faint, or "pass out," will collapse to the ground. In some folks with a hyper vagus nerve, stimulating it causes the heart to slow drastically. However, once the victim actually passes out, the vagus nerve stops doing its thing, and the victim's heart begins to speed up in order to fix the low blood pressure.


Too little water in the bloodstream lowers blood pressure, and stimulating the vagus nerve when the system is already a quart low leads 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 -- talk about a double whammy.

Psychological Triggers

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. When I taught EMT courses, students would sometimes collapse while watching a procedure in the emergency department. Other than a couple of damaged noggins from table corners, they were always fine.


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. Victims of 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 victim 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. The combination of those effects drains the brain and turns out the lights.

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, and some are serious causes of dehydration or drops in blood pressure.

  • 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 -- too much medication equals too low blood pressure


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 victim is complaining of chest pain or other symptom of a heart attack, assume the heart is too weak to keep blood in the head.

Fainting Overview


Medow, M.S., et al."Pathophysiology, diagnosis, and treatment of orthostatic hypotension and vasovagal syncope." Cardiology in review. Jan-Fed 2008 PMID: 18091397

Sheldon, R.S., et al."Age of first faint in patients with vasovagal syncope." Journal of cardiovascular electrophysiology. Jan 2006 PMID: 16426400

Sheldon, R.S., et al."Worsening of symptoms before presentation with vasovagal syncope." Journal of cardiovascular electrophysiology. Aug 2007 PMID: 17655672

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