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Aspirin About-Face

A major reversal on aspirin highlights a concept everyone should understand

David Epstein

Oct 19, 2021
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A whopping 29 million Americans — that’s the entire population of Texas — take aspirin every single day in order to prevent heart disease. Last week, the U.S. Preventive Services Task Force issued draft guidelines saying that most of those people should probably stop, because the potential harms outweigh the benefits.

That’s a big friggin’ deal. Medical recommendations change all the time, as knowledge is updated. But I think this case is a particularly teachable moment, highlighting the importance of comparing costs and benefits on the same scale. And there’s an important concept in medicine that can help with that — namely: NNT.

NNT is an abbreviation for “number needed to treat.” In other words: How many patients must be treated with the drug in order for a single patient to get the desired benefit?

When you read about drugs in the news — or even in most medical journals — you will almost never be explicitly given the NNT (which I will explain in more detail below). Instead, you’ll get relative risk reduction, a metric that a Michigan State med school dean once told me “is just another way of lying.” Why would he say that?

Relative Risk Reduction

Here’s a fictional example:

You read that a new drug reduces your chance of dying from Ryantastic syndrome by 40 percent. Here’s what that means in practice: if 10 in 100,000 people normally die from Ryantastic syndrome, and everyone takes the new drug, only 6 in 100,000 people will die from Ryantastic syndrome. Now let’s think about it from an NNT perspective.

For 100,000 patients who took the new drug, four deaths by Ryantastic syndrome were avoided, or one per 25,000 patients who took the drug. So the NNT is 25,000; that is, 25,000 patients must take the drug in order for one death-by-Ryantastic to be avoided. Ideally, you also want to know the NNH, or “number needed to harm.”

Let’s say that 1 in 1,000 patients who take the new drug suffer a particular grievous side effect. In that case, the NNH is 1,000, while the NNT is 25,000. Suddenly, the decision seems a lot more complicated than if you’re just told the drug will lower your chance of dying from Ryantastic syndrome by 40 percent.

Now let’s move to the real world: aspirin. Nearly five years ago, the NNT and NNH of aspirin caught my eye, so I included them in an article about medical evidence:

And so why did the recent task force make the new recommendation? According to the New York times:

They looked at the same kind of data that I did and saw that the tradeoff between the NNT and the NNH didn’t look so good. As a doctor I once interviewed on this topic told me: when a massive group of people who don’t have symptoms take a drug, the chances of harm will often outweigh the chances of help. That certainly is not to say that this is always the case, but as the old medical adage goes: it’s hard to make asymptomatic patients better.

Once I started looking at NNT and NNH data instead of relative risk, one of my main takeaways was that most drugs don’t do anything significantly good or bad for most people who take them. That doesn’t mean they aren’t worthwhile, it’s just a different — and, I think, important — perspective. Here’s a graphic illustration of what I mean, from my 2017 ProPublica article:

WYSIATI: “What You See Is All There Is”

The larger point I really want to hammer home is that a statistic like relative risk reduction — which is far and away the most common one you’re getting — is not the statistic that you need in order to make an informed decision.

Daniel Kahneman, the psychologist who won a Nobel for illuminating cognitive biases, writes about a bias he calls WYSIATI, or, “What You See Is All There Is.” (Pronounced <wiz-E-ah-tee>.) In his words:

The question we should ask: Here is the information that I was given; is it the information that I need in order to make a good decision?

A few years ago, I sat in on several days of a Harvard Business School class taught by Max Bazerman. (I was blown away by how engaging he was. I watched him auction off a $100 bill to HBS students for nearly $500. It was bananas.) Over the course of a two-day simulation, Bazerman showed students that they were making serious business decisions based on the information he had given them, but without asking for the information they actually needed.

On day one, he repeatedly told the students they could contact him after class if they wanted more information. When they returned on day two, Bazerman asked the class: “How many times did I say yesterday, ‘If you want additional information let me know?’” And then he answered himself. “Four times. Four times I said, ‘If you want additional information, let me know.’” Not a single student had asked for additional information.

After most of the students realized that they had botched the business decision in the simulation, Bazerman continued:

How I Apply This

When I have a major decision coming up, I try to make a quick list of the information I would want in order to feel informed. And I try to do this as early as possible, so that when I’m presented with information, I can see if something from my list is missing. (I did this recently while considering a home purchase, and found it very helpful.) It’s not perfect, of course, but I think it keeps me seeking out useful information, and helps me avoid falling prey to WYSIATI. Give it a try!

David’s Digressions

-If you’re interested in checking out NNTs for other therapies, check out The NNT, a site staffed by volunteer doctors. If you click “reviews,” and then over on the left sort by “rating,” you can check out therapies that are rated green — like defibrillators for cardiac arrest, or chewing gum after a C-section — and those that are red, like a drug that has an NNT of infinity for a given condition because it doesn’t help anyone. Some therapies that get the “Black/Caution” rating might surprise you, like vitamin D supplementation for preventing fractures in older adults.

-If you want to read more about “medical reversals” — i.e. U-turns of medical advice — the book Ending Medical Reversal has some fascinating examples, as well as suggestions for making reversals less common. More recently, two of the same authors suggested an “expiration date” by which medical evidence should be reevaluated. Of course, reevaluating in light of recent evidence is exactly what the task force that drafted the new aspirin guidelines did. So while millions of people who have been taking aspirin daily on the advice of their doctor might be upset and confused by the new recommendation, I think we ought to celebrate an effort that updated our thinking. I’m sure leaving the status quo alone would have been the path of least resistance, so kudos to the task force.

Thanks for reading. Until next week…

David

p.s. If you liked today’s post, please share it! You can find it (and other posts) here.

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29 Comments

  • Jean Arrhr
    Thank you; this is excellent. I'm a psychotherapist whose clients often have questions about medical issues; of course I tell them I am NOT a doctor, and please consult their MD and/or psychopharm, but I can give them what little I know. This will he…
    See more
    • 30w
    • Author
      David Epstein
      Jean, it's incredibly gratifying to hear — from someone who is actually doing the critical work! — that this was helpful. I hadn't even thought of that, but now that you mention it, I'm guessing you probably get medical questions of all stripes from cl…
      See more
      • 30w
  • Jan Sarbacker
    I would like to share my Aspirin experience. Several years ago when I was 65 I read where taking an aspirin a day was good so I started that, after a few days my breathing became difficult, I went to emergency room and they put me on oxygen and said I …
    See more
    • 29w
    View 2 previous replies
    • Author
      David Epstein
      Wow, Jan, I'm glad you got that figured out! Thank you for sharing your experience, and that sounds incredibly harrowing. I also appreciate your balanced perspective here, as there are clearly instances where aspirin is invaluable (like recent findings…
      See more
      • 29w
  • Marc Alexander
    David, great coverage. Have you read Risk Savvy by Gerd Gigerenzer? Dovetails nicely with what you've written here and in ProPublica.
    • 30w
    • Author
      David Epstein
      Hey Mark, indeed I have. I'm a big fan of his work, and had a chance to talk with him for a while when he visited Penn to give lectures. So I take the association you raised as a great compliment;) You also reminded me that we had an interesting conver…
      See more
      Do doctors understand test results?
      BBC.COM
      Do doctors understand test results?
      Do doctors understand test results?
      • 30w
    View 5 more replies
  • Silvana Ordoñez
    this is excellent! I can think of may situations in which I did not ask for more information but my brain worked with what was in front of me
    • 30w
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    • Author
      David Epstein
      Hey Silvana! Well, don't beat yourself up — when a cognitive bias is so famous that it has an acronym, you can trust that all of us are falling prey to it at times (if not most of the time). Perhaps being conscious of it will provide a little bit of he…
      See more
      • 30w
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  • William Murphy
    Thanks David, really enjoyed this. Like most people I've been prescribed medications where I've found it really hard to tell whether they were having any positive affect, it made me skeptical. As statistics like relative risk reduction can be so mislea…
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    • 30w
    • Author
      David Epstein
      Thanks for this note, William. I am hopeful for real improvement, as my sense is that there's a growing realization that some of this kind of thinking needs to be fundamental in medical education. (I would argue that it's fundamental in education in ge…
      See more
      • 29w
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  • Michael Gérard
    My assumption is that this speaks to taking a regular aspirin. But what about the mere 80 mg pill that used to be referred to as “baby aspirin” that small amount seems much less likely to cause a bleeding event.
    • 30w
    • Author
      David Epstein
      Hi Michael, thanks for this thought — it's an important one. I think the idea that bleeding is dose dependent is supported by the new draft guidelines. (Although they mention that analyses by dose "were limited because many cohorts reported wide ranges…
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      Draft Evidence Review: Aspirin Use to Prevent Cardiovascular Disease: Preventive Medication | United States Preventive Services Taskforce
      USPREVENTIVESERVICESTASKFORCE.ORG
      Draft Evidence Review: Aspirin Use to Prevent Cardiovascular Disease: Preventive Medication | United States Preventive Services Taskforce
      Draft Evidence Review: Aspirin Use to Prevent Cardiovascular Disease: Preventive Medication | United States Preventive Services Taskforce
      • 30w
  • Steven Kempton
    Great article David, very insightful and helpful.
    • 30w
    • Author
      David Epstein
      Appreciate the kind words Steven...and it's helpful feedback here in my early Bulletin newsletter days.
      • 30w
  • Linda Guillory
    To all of the people out there taking ASA talk to your doctor dont reply on face book. For heaven sakes Talk to your Doctor !!!!!!
    • 30w
    • Author
      David Epstein
      Thanks Linda! I appreciate that concern. And as I'm sure is the case for your own reply here, comments left on posts needn't be in zero-sum competition with talking to your doctor;)
      • 30w
  • Adam Conley
    Yeah but if the gastro bleeding isn't all that bad, or even if it is, that's easier to treat than a heart attack. Not to mention even if they both can kill you, ones easier to survive and the long term effects of a heart attack are way worse than the l…
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    • 29w
    • Author
      David Epstein
      Hey Adam, I appreciate this perspective; you're looking at this in a really nuanced way. I would add that the research the task force cited showed that aspirin did not prevent fatal heart attacks in people who were taking it and did not have heart dise…
      See more
      Homepage – TheNNT
      THENNT.COM
      Homepage – TheNNT
      Homepage – TheNNT
      • 29w
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