For some, the sight of blood — whether from a paper cut or during a blood draw — can trigger an unexpected and intense reaction: fainting. This response, known as vasovagal syncope, is common, and it’s rooted in how your body reacts to stress.

“Fainting, or syncope, occurs when there’s a temporary drop in blood flow to the brain. This can result from a sudden decrease in heart rate or blood pressure, leading to a brief loss of consciousness,” says Akhil Narang, MD, a cardiologist at Northwestern Medicine.

“Recognizing the early warning signs of fainting is important to prevent injury from falling,” he says. “Most fainting episodes themselves aren’t dangerous, but the associated trauma can be.”

What Is Vasovagal Syncope?

This overreaction causes your blood pressure and heart rate to drop suddenly. As a result, blood vessels widen, and blood flow to the brain decreases. Without enough blood reaching your brain, you may feel lightheaded, dizzy or even lose consciousness temporarily.

Who Faints?

Syncope affects 1 million Americans every year. It accounts for 3% of all emergency department visits and 6% of all hospital visits. “Approximately one-third of us will experience fainting due to syncope at least once in our lifetime,” says Dr. Narang.

“Fainting can occur at any age, but vasovagal syncope tends to be more common in younger individuals and those who are otherwise healthy,” says Dr. Narang. “Dehydration, prolonged standing or emotional stress can increase the likelihood of fainting.”

Myths about fainting are common, including that the person has epilepsy or cardiac disease. But Dr. Narang cautions that these are misconceptions.

“While fainting can occasionally signal an underlying heart problem, many cases, especially vasovagal syncope, may not be related to the heart,” he says.

“Another myth is that fainting only happens to the elderly, when in fact it’s often seen in younger individuals as well,” he adds.

Some people may have a genetic predisposition to fainting, but Dr. Narang says that most cases are situational and related to the body’s autonomic response rather than inherited factors.

Why Does the Sight of Blood Trigger It?

The vagus nerve is sensitive to emotional and physical stress. For some people, the sight of blood can be enough to trigger this nerve. It’s not just fear — it’s a physical response that your body uses to cope with overwhelming stimuli.

Common triggers include:

• Seeing blood or injury
• Standing for long periods
• Suddenly standing up
• Hunger or dehydration
• Sudden fright or emotional stress
• Severe stress or fatigue

What Is ‘Feeling Faint’?

When you start to feel faint, you may experience some warning signs, such as:

• Lightheadedness
• Dizziness
• Sweating or sweaty palms
• Tunnel vision
• Ringing in the ears
• Nausea or general weakness
• Pale skin color

“Typically, symptoms develop over seconds to a minute,” says Dr. Narang. “These symptoms are your body’s way of signaling that blood flow to the brain is dropping.”

What Are the Warning Signals?

If your body is sending warning signals that fainting is imminent, take immediate action to avoid injuring yourself.

“Lie down with your legs elevated if possible, or sit and place your head between your knees,” Dr. Narang advises. “This helps restore blood flow to the brain.”

Loosen any tight clothing and take slow, deep breaths.

“You also can perform counter-pressure maneuvers like crossing your legs, tensing your arms or squeezing a stress ball to increase blood pressure. These techniques can sometimes stop a fainting spell,” he says.

What Should You Do After Fainting?

If you do faint, remain lying down for 10 or 15 minutes once you wake up. Check to see if you have a significant injury, such as a bump on your head or a hip injury. Also try moving your legs and then get up slowly. Don't drive until your doctor feels that it is safe to do so.

“After an episode, most people recover quickly, but it’s possible to feel lightheaded or unsteady for several minutes. Standing too soon can trigger another episode, so it’s best to rest and rise gradually. If you are dehydrated, you should also drink fluids,” says Dr. Narang.

Always let your care team know if you fainted.

Is Fainting Dangerous?

Most cases of vasovagal syncope are not life-threatening.

You’ll want to rule out underlying causes, especially if fainting happens often or without warning.

Talk to your healthcare team. They may recommend tests to check your heart and nervous system. Tests may include:

• Blood tests
• Electrocardiogram
• Echocardiogram
• Tilt table testing
• Ambulatory heart monitor
• Implantable loop recorder

Can Fainting Be Prevented?

There are no universal medications to prevent vasovagal syncope.

“In recurrent or severe cases, physicians might use agents that raise blood pressure or modulate the nervous system response, but treatment is individualized,” explains Dr. Narang.

He recommends lifestyle measures, such as staying hydrated, adjusting salt intake (if directed by your doctor) and avoiding triggers, as first-line strategies in preventing fainting.

Other ways to prevent fainting:

• Eat and drink water regularly.
• Use compression stockings.
• Practice exposure therapy if blood or medical procedures cause anxiety.

Exposure therapy is a form of cognitive behavioral therapy that helps people gradually face their fears in a safe environment. Over time, exposure therapy with a trained therapist can reduce your body’s stress response and help prevent fainting.

When to Seek Help

For most people, fainting lasts less than a minute. If the person is unconscious for a longer time, call 911.

Likewise, if you hit your head or suffer serious injury from falling, seek medical attention immediately.

“If fainting is recurrent or occurs without warning, it’s important to get evaluated to exclude a cardiac cause. However, in most cases, education, reassurance and awareness of triggers go a long way in reducing recurrence,” says Dr. Narang.