What Triggers the Vagal Response?

Why someone may unexpectedly faint

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The vagal response (vasovagal reflex) is when stimulation of the vagus nerve causes symptoms such as lightheadedness, sweating, and blurred vision. This can happen because of stress, pain, heat, having a bowel movement, or even standing too long.

In some cases, vasovagal syncope—a typically brief episode of passing out—can also occur.

This article explains the vagal response and its causes. It discusses symptoms you may experience, how a vagal response episode is treated, and some ways you can prevent one from occurring.

What Is the Vagus Nerve?

The vagus nerve is the longest nerve in the body. It runs from the brain stem to the chest and abdomen.

Vagal Response Causes

The vagal response is a neurocardiogenic response. This means it involves your central nervous system (brain and spinal cord), peripheral nervous system (nerves), and cardiovascular system (heart and blood vessels).

The vagal response causes your blood pressure and heart rate to drop suddenly. When this happens, the blood vessels in your legs may dilate, or widen. This can cause blood to pool in your legs, which can cause your blood pressure to drop even further.

All of these changes can result in less blood flow to your brain.

The vagal response is a normal function of the human body, although some people are more likely to experience it than others. It is also more likely in people with certain underlying health conditions, including heart or nervous system disorders.

Vasal Response Triggers

There are a variety of triggers that can set off the vagal response. Some are internal, while others come from the environment.

Common triggers of the vagal response include:

Some people may experience vasovagal episodes as a side effect of medication. Be sure to discuss the possibility with your healthcare provider.

Symptoms of the Vagal Response

Once a vagal response has been triggered, you may have a variety of physical symptoms. These include:

The strength of the vagal response and the resulting symptoms vary from person to person.

vagal response symptoms

Verywell / Brianna Gilmartin

Vasovagal Syncope

Vasovagal syncope is a loss of consciousness triggered by a vagal response. With this, a person faints due to a drop in blood pressure and blood flow to the brain.

Vasovagal syncope can cause a person to pass out. The loss of consciousness typically only lasts for a couple of minutes.

As blood flow returns to the brain, the person will wake up and return to normal consciousness.

A vagal response that causes syncope isn't usually serious. It can, however, be the sign of a serious condition like heart arrhythmias or even a sudden heart attack. It also can lead to serious injury from a fall.

What To Do During a Vasovagal Episode

A vasovagal episode is not usually a sign of a health emergency. Some things that may help resolve the episode include:

  • Lying down to improve blood flow to the brain
  • Positioning your head between your knees
  • Drinking some water
  • Not standing up too quickly, so as to avoid the risk for fainting

Vagal Response During Bowel Movements

Some people, especially those with irritable bowel syndrome (IBS), have vasovagal symptoms when they have a bowel movement. To prevent this from happening, try to keep yourself relaxed. Sit on the toilet with your head down and your legs crossed. This may help to keep your blood pressure steady.


The vagal response can't always be prevented, but some measures may help to ward off episodes. These include:

  • Getting good sleep
  • Staying well hydrated
  • Avoiding standing for extended periods of time
  • Dressing in light, breathable fabrics so that you don't get overheated


The vagal response is a series of unpleasant symptoms that occur when the vagus nerve is stimulated. Often, this response is triggered by certain things like stress, pain, and fear.

Symptoms of the vagal response include dizziness, nausea, ringing ears, and sweating. In some cases, it can make you pass out. This is called vasovagal syncope.

If you experience a vagal response, lie down for a few minutes or sit and place your head between your knees, and avoid standing quickly. Always tell your healthcare provider about these symptoms so that they can check for any other conditions.

6 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.
  1. Cedars-Sinai. Vasovagal Syncope.

  2. Raj S, Sheldon R. Management of postural tachycardia syndrome, Inappropriate sinus tachycardia and vasovagal syncopeArrhythm Electrophysiol Rev. 2016;5(2):122-9. doi:10.15420/AER.2016.7.2

  3. UpToDate. Patient education: Syncope (fainting) (Beyond the Basics).

  4. Blanc JJ. Syncope: Definition, epidemiology, and classification. Cardiol Clin. 2015;33(3):341-5. doi:10.1016/j.ccl.2015.04.001

  5. National Institute of Neurological Disorders and Stroke. Syncope Information Page.

  6. Johns Hopkins Medicine. Syncope (Fainting).

Additional Reading

By Barbara Bolen, PhD
Barbara Bolen, PhD, is a licensed clinical psychologist and health coach. She has written multiple books focused on living with irritable bowel syndrome.