Why Seeing Blood Makes People Faint

Summary

Chasing Life: Why Seeing Blood Makes People Faint

Podcast: Chasing Life
Host: Dr. Sanjay Gupta (CNN Podcasts)
Episode Date: October 28, 2025

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Episode Overview

This episode, timed for Halloween, addresses why some people have such strong aversions to blood that they faint at the mere sight of it. Dr. Sanjay Gupta answers a listener's question by breaking down the neurobiology of blood-injury-injection phobia (BII), including its unique physiological response and potential evolutionary roots. The episode also addresses the safety of artificial sweeteners, featuring commentary from an expert in health policy. Throughout, Dr. Gupta emphasizes practical advice, evidence-based insights, and the importance of asking questions about our health.

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Key Discussion Points & Insights

1. Why Do Some People Faint at the Sight of Blood?

• Listener Question from Heidi:
  "What happens in the brain to make some people fear blood and others don't?"
  (00:29)

Dr. Gupta’s Breakdown (02:33)

• Hemophobia Defined:

  • Extreme fear of blood, also called hemophobia, is part of blood-injury-injection phobia (BII), affecting about 3–4% of people.
  • BII encompasses fear of blood, needles, injuries, and medical procedures.

• Unique Physiological Phases:

  • Most phobias (e.g., spiders): Trigger the sympathetic "fight or flight" response—increased heart rate and blood pressure.
  • BII response:
    • Phase 1: Brief hyperarousal (heart rate and blood pressure spike).
    • Phase 2: Dramatic crash in both (vasovagal response).
    • This sudden drop decreases blood flow to the brain, causing fainting.
    • Controlled by overstimulation of the vagus nerve.

  "A BII response, on the other hand, sort of seems to have two phases. First, a short burst of hyperarousal... followed by a dramatic crash ... That is why people faint, because there is a sudden decrease in blood flow to the brain."
  — Dr. Gupta (03:20)

• Brain Differences:

  • People with BII:
    • More activity in vision and attention parts of the brain (thalamus, occipital temporal cortex).
    • Less amygdala (fear/emotion center) activity compared to other phobias.
  • 2009 study: BII phobics’ neural patterns differ significantly from spider phobics.

• Why Some People and Not Others?

  • Genetics may play a role.
  • Higher prevalence among women noted in some studies.
  • Possible evolutionary purpose: “Fainting and then falling horizontally is a way to restore blood flow to the brain in case of major blood loss.” (04:33)

Practical Coping Tip: The Applied Tension Technique

• How to Use:
  1. Sit comfortably.
  2. Tense muscles in arms, legs, and trunk for ~10 seconds (until your head feels slightly warm).
  3. Relax for 20–30 seconds.
  4. Repeat five times.
• Purpose: Raises blood pressure a bit to prevent fainting.
• Counter-intuitive Advice: Don’t simply try to relax—too much relaxation might lower blood pressure further and increase fainting risk (05:32).

"While the goal of many phobia-related therapies is to help you relax, that's not necessarily the goal here... Feeling too relaxed might mean your blood pressure is even lower..."
— Dr. Gupta (05:45)

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2. Artificial Sweeteners: Are They Safe? (07:39)

• Listener Question from Ashish:
  “How safe are the sugar substitutes...Aspartame, or various other names? If consumed in moderation, do they have any side effects?”
  (07:39)

Dr. Gupta’s Overview (08:05)

• Types of Sweeteners:

  • Synthetic (aspartame, sucralose, saccharin).
  • “Natural” (stevia from stevia plant).
  • All FDA-approved.

• Pros:

  • Low/no calories
  • Do not cause tooth decay.
  • Usually do not raise blood sugar—useful for people with diabetes or weight loss goals.

"They are low calorie or no calorie, they do not cause tooth decay like sugar can, and they usually don't raise blood sugar levels..."
— Dr. Gupta (08:14)

Expert Perspective: Dr. Laura Schmidt (09:13)

• American vs. Precautionary Principle:

  • U.S. often tests product safety after public exposure ("put it in the cereal and wait to see if it's safe").
  • Cautions that long-term safety isn't always assessed before products go to market.

• Potential Drawbacks & Debates:

  • Some major studies show negative effects on the gut microbiome.
  • Research links consumption to weight gain/obesity—though debate exists over causation.
  • Multiple studies suggesting risks mean “we ought to be thinking twice about it.” (10:32)

“There are some very influential papers...that show very negative effects on the gut microbiome... consumption ... actually increases your weight, can produce obesity.”
— Dr. Laura Schmidt (10:06)

Dr. Gupta’s Takeaway (10:42)

• Helpful for “soda addictions” to quit sugary sodas.
• If you don’t have diabetes, best to avoid artificial sweeteners.
• Moderation if you use them.
• Healthiest sweet snack? Whole fruit—enjoy it slowly and mindfully.

“If you can avoid artificial sweeteners, it's probably a good idea... the healthiest snack option if you're looking for something sweet is whole food, like a fruit. Try it. Eat it slowly.”
— Dr. Gupta (11:12)

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Notable Quotes & Memorable Moments

• On the evolutionary theory behind fainting:
  "Fainting and then falling horizontally is a way to restore blood flow to the brain in case of major blood loss. I thought that was kind of fascinating."
  — Dr. Gupta (04:33)

• On therapy for hemophobia:
  "While the goal of many phobia related therapies is to help you relax, that's not necessarily the goal here. Right, because feeling too relaxed might mean that your blood pressure is even lower..."
  — Dr. Gupta (05:45)

• On American food safety standards:
  "In America, we don't follow the precautionary principle... Instead, we flip it around and we say, put it in the cereal and wait to see if it's safe."
  — Dr. Laura Schmidt (09:21)

• On choosing natural sweetness:
  "Eat it slowly. Taste every molecule of it. It's pretty good."
  — Dr. Gupta (11:14)

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Important Segment Timestamps

• What makes some people fear blood? (02:33–06:27)
• How to prevent fainting at sight of blood (05:18–05:55)
• Artificial sweeteners explained (08:05–09:12)
• Expert on food chemicals – Laura Schmidt (09:13–10:42)
• Dr. Gupta's final take & practical advice (10:42–11:30)

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Episode Tone & Closing Thoughts

The episode is upbeat, scientific, and reassuring, reflecting Dr. Gupta's curiosity and commitment to practical, evidence-based health guidance. He encourages listeners to keep asking questions about health—as “part of chasing life.” His advice is both compassionate and actionable, demystifying complex topics with clarity and warmth.

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For those who haven’t listened, this episode offers an accessible explanation of why some of us faint at the sight of blood and provides valuable tips for managing both blood-related fears and choices about artificial sweeteners. The practical advice and expert insights make it a worthwhile listen—especially before Halloween!

Transcript

[A]

Hey, there. Welcome to Paging Dr. Gupta. This is the place where you get to ask your health questions, share your concerns, and hopefully get some real answers, some real news you can use. Our producer Jesse is with us today with some brand new questions from the inbox. Jesse, who's up first?

[B]

Hey, Sanjay. We got a couple of questions we thought might be fun with Halloween being this week. The first is from Heidi in California. Check it out.

[C]

I've always wondered this. What happens in the brain to make some people fear blood and others don't? For example, I can't really get my blood drawn. Or if I cut myself, I get queasy. Sometimes I pass out, sometimes I curl up. But then I have a friend or a family member who can look at themselves while they get their blood drawn. And they don't share blood at all. So what happens with the brain that makes some fear it and some don't?

[A]

All right, thank you so much, Jessi, and thank you, Heidi. Perfect timing with this question with Halloween coming up, my favorite holiday. Yes, blood is everywhere this week, but some of us react way more strongly than others. And I love this question because what it fundamentally does is get to the heart of how our brains and bodies process certain fears and anxieties. So we're gonna have some fun, and I'm gonna break down the neurobiology for you right after the break.

[C]

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[A]

We are talking about why some people fear blood so much that the sight of it can make them pass out. Well, first things first. An aversion to blood is normal for just about all humans. That's why horror movies are so successful. But the extreme fear of blood that Heidi is asking about actually has a name. It's called hemophobia. HEM stands for blood, phobia stands for fear. It is part of a broader category called blood injury injection phobia. BII. Now, BII affects about 3 to 4% of the general population. It includes a fear of needles and injuries and medical procedures. It is sort of a unique phobia for a few reasons. If you think about most phobias, like arachnophobia, that's a fear of spiders. What happens is just seeing the trigger. Seeing the spider, in this case, causes a typical sympathetic hyperarousal. In other words, fight or flight, increased heart rate, blood pressure. A BII response, on the other hand, sort of seems to have two phases. First, a short burst of hyperarousal, a temporary surge in heart rate and blood pressure, like other phobias. But that phase is then followed by a dramatic crash in both blood pressure and heart rate. And that is why people faint, because there is a sudden decrease in blood flow to the brain. It occurs when the vagus nerve, which controls, among other things, heart rate and blood pressure, is overstimulated. Now, this unique physical reaction triggered by BII suggests that the brain processes this differently from other phobias. And brain scans have backed this up. There was a study from 2009 which compared how people with BII reacted to seeing blood versus how people with arachnophobia reacted to seeing spiders. So when the spider phobic people saw spiders, their brains lit up, and in the amygdala, that's the brain's central emotional and fear hub. But that did not happen for people with bii. Instead, their brain showed more activity in areas tied to vision and attention, like the thalamus and parts of the occipital temporal cortex. Now, answering exactly why hemophobia impacts some people and not others, that can be tricky. There might be a genetic link. Sex might play a role. Some studies have found a higher prevalence among women. Some people have theorized that fainting is, in fact, an evolutionary defense mechanism. The idea being that fainting and then falling horizontally is a way to restore blood flow to the brain in case of major blood loss. I thought that was kind of fascinating. But, Heidi, whatever the reason, if you have hemophobia or bii, here's a trick that can really help you. The applied tension technique. So, sit in a comfortable chair and tense the muscles in your arms, legs and trunk for about 10 seconds. Hold the tension until your head starts to feel a little warm. Then relax your body for 20 to 30 seconds. Repeat that five times. That'll increase your blood pressure a little bit and hopefully prevent fainting. Might be a good idea to practice this a few times a day ahead of the appointment. And remember, while the goal of many phobia related therapies is to help you relax, that's not necessarily the goal here. Right, because feeling too relaxed might mean that your blood pressure is even lower, which would make you even more likely to faint. Alright, time to take a quick break. But it wouldn't be Halloween without talking about sugar. Coming up, I'll tell you about the potential pitfalls of artificial sweeteners. We'll be right back.

[E]

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[A]

And we are back with paging Dr. Gupta. Jesse, who do we have next?

[B]

Okay, our next question is from Ashish in Florida.

[G]

Hi Dr. Gupta, this is Ashish. How safe are the sugar substitutes that are available in the market? There are some sugar substitutes with very difficult names to pronounce. Aspartame or various other names. How safe they are to consume, of course, if consumed in moderation. And do they have any side effects that one needs to be worried about?

[A]

All right, thank you for your question, Ashish. Let's start by breaking down what sugar substitutes, artificial sweeteners, what they really are and why some people might want to use them. So first of all, the good news, they are low calorie or no calorie, they do not cause tooth decay like sugar can, and they usually don't raise blood sugar levels, which is why they're sometimes used by people with diabetes or people trying to lose weight. Now, many of them are synthetic sugar substitutes, right? Synthetic meaning that they are made from synthesizing chemicals, which is why they have those hard to pronounce names. There are a few common types, for example, aspartame brand names are NutraSweet and Equal. You've probably had it in diet sodas like Diet Coke. There's sucralose, also known as Splenda, saccharin, often used in baked goods, jams, jellies, candy. Then there are some sugar substitutes that are billed as being more natural, such as stevia, which is extracted from the leaf of the stevia plant. Now, the FDA allows the substitutes I just mentioned, along with a handful of others.

[H]

The problem with the chemical additives in our food supply, which are now a topic of major concern in the field, is that in America, we don't follow the precautionary principle, which would say, before we put this chemical in cereal that a child eats every morning, we should be thinking about whether it's safe. Instead, we flip it around and we say, put it in the cereal and wait to see if it's safe.

[A]

That's Laura Schmidt, a professor of health policy in the School of Medicine at the University of California at San Francisco. She is a leading expert in the field of sugar studies. I talked with her on a past episode about this exact topic and her advice was stick to the World Health Organization's guidelines around artificial sweeteners, which is basically this. If you don't have diabetes, stay away.

[H]

When it comes to artificial sweeteners, there's science on both sides. There are some very influential papers and published in major journals like Nature that show very negative effects on the microbiome, the gut microbiome. And there are studies that show that consumption of these products actually increases your weight, can produce obesity. And there's a lot of debate about what's going on there, is that reverse causation or not? But I think if there are multiple studies suggesting that something's potentially harmful, we ought to be thinking twice about it.

[A]

Now, I will say on one hand, there have been studies that have shown artificial sweeteners can be helpful for people with soda addictions who want to quit drinking sugary sodas. But Ashish, if you've listened to this podcast before, you've probably also heard me talking about the precautionary principle, and I think as a result, Laura's guidance here is sound. If you can avoid artificial sweeteners, it's probably a good idea if you do decide to give a sugar substitute a try. Moderation is key and not to be a total buzzkill here, but I should also remind you that the healthiest snack option if you're looking for something sweet is whole food, like a fruit Try it. Eat it slowly. Taste every molecule of it. It's pretty good. Thank you for all your fantastic questions. Thank you for staying curious. Asking questions about your health is never a bad thing. It's why we set up this podcast. It's one of the best things you can do for yourself. Part of chasing life is asking questions, the big ones, the small ones. So keep searching, keep learning. And if you've got a question you've been wondering about, send it our way. We might just answer it on our next show. Record a voice memo, email it to asksanjayn.com or give us a call 470-396-0832 and leave a message. Thanks for listening.

[B]

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