Why Do I Get Weak When I See Blood? Vasovagal Syncope

Medically Reviewed on 4/12/2022
Why Do I Get Weak When I See Blood
For some people, seeing blood can cause them to go weak or even faint due to a sudden drop in heart rate or blood pressure

Most people feel uneasy at the sight of blood. For some people, however, seeing blood can cause them to go weak or even faint due to a sudden drop in heart rate or blood pressure

This sudden drop in blood pressure is called the vasovagal response.

What is vasovagal syncope?

Vasovagal syncope (VVS) or reflex syncope is a sudden drop in the heart rate or blood pressure that occurs when the body overreacts to triggers such as the sight of blood or extreme emotional distress.

Vaso refers to the blood vessels, vagal refers to the vagus nerve, and syncope refers to a loss of consciousness.

The vagus nerve helps control the speed and force of the heartbeat, so when stress is put on the vagus nerve, it can short circuit, causing the heart rate to slow down and blood vessels to widen, causing blood to pool in the legs and compromising the blood flow to the brain. This can lead to loss of consciousness.

Vasovagal syncope is usually not dangerous, but it can be if losing consciousness causes you to fall from a height or hit your head. Immediate medical help should be sought if injuries occur during fainting.

What are associated symptoms of vasovagal syncope?

Some people do not notice any warning signs before fainting at the sight of blood. However, others may have symptoms such as:

What to do if you feel weak at the sight of blood

While you may not be able to prevent feeling weak at the sight blood, there are things you can do to reduce the likelihood of fainting:

  • Lie down. If you feel dizzy, lie down with your head level with your heart to ensure ample blood flow to the brain. The best position is lying down next to a wall with your legs propped up against it to get the blood flowing to the brain.
  • Tense your muscles. This is called the applied tension technique, which involves tensing your muscles in your body for about 10-15 seconds at a time to raise your blood pressure.
  • Anticipate triggers. Anticipate situations that may trigger symptoms, such as having blood drawn. Make sure to lie down for the procedure.
  • Stay hydrated. Though staying hydrated may not prevent the vagus nerve from firing altogether, it may help the brain be less sensitive to low blood pressure, reducing the risk of fainting.

What to do after fainting from the sight of blood

If you have fainted, you may feel weak, tired, and nauseated when regaining consciousness. Make sure to get plenty of rest and drink water throughout the day.

In some cases, you may need to seek emergency medical attention after a fainting episode, particularly if you experience additional symptoms that overlap with signs of a heart attack.

How is vasovagal syncope managed?

Medications are not usually needed for a single episode of vasovagal syncope. However, in certain circumstances, the following medications can effectively reduce the frequency of fainting episodes:

  • Alpha-1 agonists: Help raise blood pressure
  • Fludrocortisone: Help maintain blood pressure by increasing sodium and fluid levels in the body
  • Beta-blockers: Useful in people who are over the age of 42


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Medically Reviewed on 4/12/2022
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Cedars-Sinai. Vasovagal Syncope. https://www.cedars-sinai.org/health-library/diseases-and-conditions/v/vasovagal-syncope.html

Jeanmonod R, Sahni D, Silberman M. Vasovagal Episode. [Updated 2021 Oct 7]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022. https://www.ncbi.nlm.nih.gov/books/NBK470277/