What Causes Fainting?

Fainting can be a symptom of several health conditions, as well as the result of low blood pressure and/or dehydration.


If you think you are going to faint, it’s important to sit or lie down (on the floor if necessary) to avoid falling and injuring yourself.

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The medical term for fainting is syncope, and it refers to a sudden loss of consciousness that can happen due to hypoxia (oxygen deprivation in the brain). If the levels of blood and oxygen in your brain decrease for any reason, your brain reroutes blood and oxygen from non-vital parts of your body to your organs, to ensure that they keep functioning as they should. Your brain also tells your heart rate to rise and your breathing to increase, in an attempt to get more oxygen circulating to your brain. This results in low blood pressure elsewhere in your body, and this may lead to a brief, sudden loss of consciousness.

Women and older adults are at greater risk for fainting, and research suggests that it is the reason for up to 3 percent of emergency department visits. Often fainting doesn’t indicate any serious medical problems, but if you’ve been fainting you should mention it to your doctor, particularly if you’ve noticed that your heart tends to race before you faint, or you feel a squeezing sensation in your chest. That’s because it’s possible for fainting to point to a serious underlying heart condition, such as a heart arrhythmia, or heart attack. Fainting also can suggest diabetes, or a stroke.

Fainting Causes

The causes of fainting can include:

  • Heart problems, such as abnormal heart rhythms, narrowing (stenosis) of the heart valves, aortic dissection (in which the inner lining of the aortic wall tears, allowing blood to enter and split the layers of the aortic wall), heart attack, heart failure, and high blood pressure.
  • Autonomous nervous system problems. The autonomous nerve system influences involuntary actions, such as your heart rate, breathing, and the speed at which you digest your food. If the autonomous nerve system malfunctions for any reason, you may faint. Common culprits for autonomous nerve system malfunction include a sudden shock, seeing something unpleasant (for example, you may have a phobia about blood), becoming overheated, and standing up for a long period of time. Rarely, simple actions such as coughing, sneezing, or lifting something heavy also can cause this type of fainting.
  • Low blood pressure. A phenomenon called orthostatic hypotension can occur if you stand up suddenly after being seated or prone. As you rise, your blood rushes to your lower limbs and the supply of blood and oxygen to your brain plummets. Several factors can make you more susceptible to this type of fainting, including dehydration (which lowers blood pressure), diabetes, particularly if it is not diagnosed or you are not following your doctor’s advice for managing high blood sugar. Certain medical conditions (such as Parkinson’s disease) and medications also are implicated—prime examples include drugs that treat high blood pressure, such as diuretics, angiotensin-converting enzyme (ACE) inhibitors, angiotensin receptor blockers (ARBs), and beta-blockers. You also are at greater risk for this type of fainting if you overindulge in alcohol in a brief period of time.

Fainting Signs

If you faint frequently, it’s a good idea to get clued into the signs that a fainting spell might be about to happen. Fainting signs include:

  • Blurred vision
  • Dizziness/lightheadedness
  • Nausea
  • Yawning
  • Excessive perspiration
  • Feeling too hot
  • Feeling as if your legs are very heavy

Fainting Treatment

Your doctor may refer you for various tests to try to find the underlying cause of your fainting. For example, you may have an electrocardiogram to check for an abnormal heart rhythm and/or poor blood flow through your heart. You also may have blood tests.

If a medical condition is causing your fainting spells, treating the condition should help to alleviate the problem. If you are taking medications that are associated with a greater risk of fainting, your doctor may modify your dose or substitute a drug that doesn’t cause fainting. If you take high blood pressure medications, it’s a good idea to take the precaution of rising slowly from a sitting or prone position. If your job requires long periods of standing, try to take regular breaks from being on your feet, and make sure you rehydrate and eat snacks.

If you think you are going to faint, it’s important to sit or lie down (on the floor if necessary) to avoid falling and injuring yourself. Putting your head between your knees may help too, and some people find that countermeasures like fisting their hands very tightly, can ward off a fainting spell. If you see somebody else faint, check that they are breathing. If not, perform CPR. If they are breathing, loosen any restrictive clothing, such as ties, shirt collars, and belts, and place something under their feet to raise their lower legs and ensure that blood flows back towards their brain. If the person does not regain consciousness within a minute, call 9-1-1.

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Kate Brophy

Kate Brophy is an experienced health writer and editor with a long career in the UK and United States. Kate has been Executive Editor of the Icahn School of Medicine … Read More

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