Aspirin for Primary Prevention — What We Really Know

This transcript has been edited for clarity. 
Aspirin. Once upon a time, everybody over age 50 years was supposed to take a baby aspirin. Now we make it a point to tell people to stop. What is going on? I’m Christopher Labos, and this is On Second Thought. 
Our recommendations vis-à-vis aspirin have evolved at a dizzying pace. The young’uns watching us right now don’t know what things were like in the 1980s. The Reagan era was a wild, heady time where nuclear war was imminent and we didn’t prescribe aspirin to patients. 
That only started in 1988, which was a banner year in human history. Not because a number of doves were incinerated by the lighting of the Olympic torch at the Seoul Olympics – look it up if you don’t know what I’m talking about – but because 1988 saw the publication of the ISIS-2 trial, which first showed a mortality benefit to prescribing aspirin post–myocardial infarction (MI).
Giving patients aspirin during or after a heart attack is not controversial. It’s one of the few things in this business that isn’t, but that’s secondary prevention – treating somebody after they develop a disease. Primary prevention, treating them before they have their incident event, is a very different ballgame. Here, things are messy. 
For one thing, the doses used have been very inconsistent. We should point out that the reason for 81 mg of aspirin is very arbitrary and is rooted in the old apothecary system of weights and measurements. A standard dose of aspirin was 5 grains, where 20 grains made 1 scruple, 3 scruples made 1 dram, 8 drams made 1 oz, and 12 oz made 1 lb – because screw you, metric system. Therefore, 5 grains was 325 mg of aspirin, and 1 quarter of the standard dose became 81 mg if you rounded out the decimal. 
People have tried all kinds of dosing structures with aspirin prophylaxis. The Physicians’ Health Study used a full-dose aspirin, 325 mg every 2 days, while the Hypertension Optimal Treatment (HOT) trial tested 75 mg daily and the Women’s Health Study tested 100 mg, but every other day. 
Ironically, almost no one has studied 81 mg every day, which is weird if you think about it. The bigger problem here is not the variability of doses used, but the discrepancy when you look at older vs newer studies.
Older studies, like the Physicians’ Health Study, did show a benefit, at least in the subgroup of patients over age 50 years, which is probably where the “everybody over 50 should be taking an aspirin” idea comes from, at least as near as I can tell. 
More recent studies, like the Women’s Health Study, ASPREE, or ASPIRE, didn’t show a benefit. I know what you’re thinking: Newer stuff is always better. That’s why you should never trust anybody over age 40 years. The context of primary prevention studies has changed. In the ’80s and ’90s, people smoked more and we didn’t have the same medications that we have today. We talked about all this in the beta-blocker video to explain why beta-blockers don’t seem to have a benefit post-MI.
We have a similar issue here. The magnitude of the benefit with aspirin primary prevention has decreased because we’re all just healthier overall. So, yay! Progress! Here’s where the numbers matter. No one is saying that aspirin doesn’t help. It does. 
If we look at the 2019 meta-analysis published in JAMA, there is a cardiovascular benefit. The numbers bear that out. I know you’re all here for the math, so here we go. Aspirin reduced the composite cardiovascular endpoint from 65.2 to 60.2 events per 10,000 patient-years; or to put it more meaningfully in absolute risk reduction terms, because that’s my jam, an absolute risk reduction of 0.41%, which means a number needed to treat of 241, which is okay-ish. It’s not super-great, but it may be justifiable for something that costs next to nothing. 
The tradeoff is bleeding. Major bleeding increased from 16.4 to 23.1 bleeds per 10,000 patient-years, or an absolute risk increase of 0.47%, which is a number needed to harm of 210. That’s the problem. Aspirin does prevent heart disease. The benefit is small, for sure, but the real problem is that it’s outweighed by the risk of bleeding, so you’re not really coming out ahead. 
The real tragedy here is that the public is locked into this idea of everyone over age 50 years should be taking an aspirin. Even today, even though guidelines have recommended against aspirin for primary prevention for some time, data from the National Health Interview Survey sample found that nearly 1 in 3 older adults take aspirin for primary prevention when they shouldn’t be. That’s a large number of people. That’s millions of Americans — and Canadians, but nobody cares about us. It’s fine. 
That’s the point. We’re not debunking aspirin. It does work. The benefits are just really small in a primary prevention population and offset by the admittedly also really small risks of bleeding. It’s a tradeoff that doesn’t really work in your favor.
But that’s aspirin for cardiovascular disease. When it comes to cancer or DVT prophylaxis, that’s another really interesting story. We might have to save that for another time. Do I know how to tease a sequel or what? 
For Medscape, I’m Christopher Labos.
Christopher Labos, MD, MSc, is a cardiologist with a degree in epidemiology. He spends most of his time doing things that he doesn’t get paid for, like research, teaching, and podcasting. Occasionally he finds time to practice cardiology to pay the rent. He realizes that half of his research findings will be disproved in 5 years; he just doesn’t know which half. He is a regular contributor to the Montreal Gazette, CJAD radio, and CBC and CTV television in Montreal and is host of the award-winning podcast The Body of Evidence. He is also the author of Does Coffee Cause Cancer?, a book about food myths disguised as a romantic comedy.