Aspirin bleeding risk 'balances out' lower heart attack risk in 'worried well'

"Daily dose of aspirin 'not worth risk' as study warns of bleeding side-effects," reports The Daily Telegraph.

Aspirin slows the ability of the blood to form clots, which means it can reduce the chances of dangerous blood clots causing heart attacks and strokes. However, the same action means it increases the risk of serious bleeding, such as bleeding of blood vessels in the brain or gut. And in some cases this type of bleeding can be as life-threatening as a heart attack or stroke.

The balance of risks and harms associated with aspirin use has been debated for years. For people who have had a heart attack or stroke, the benefit of daily low-dose aspirin to prevent another one outweighs the bleeding risk.

The picture is less clear for healthy people. In the UK, people are not advised to take aspirin in the hope of preventing a first heart attack or stroke. But many do so, and guidelines in other countries differ.

In this study UK researchers reviewed all the current evidence on the subject. After pooling data, researchers estimated that for every 265 people who took aspirin, only 1 would benefit from preventing a heart attack or stroke. On the other hand, for every 210 people who took aspirin, 1 would experience a serious bleeding event.

The results suggest people should continue to follow UK guidelines and only take daily low-dose aspirin to prevent a heart attack or stroke if their doctor has recommended it.

Where did the story come from?

The researchers who carried out the study were from King's College London. No funding information was made available. The study was published in the peer-reviewed Journal of the American Medical Association.

The Mail Online and The Daily Telegraph carried balanced and accurate reports of the study, though both decided to use the more dramatic statistical results. They report that the risk of heart attack or stroke is 11% lower with aspirin, but this is relative to people not taking aspirin. They do not explain that the number of people included in the review who had a heart attack or stroke was very small. So the actual reduction in risk is only 0.38%. It is a similar story for bleeding risk – 44% higher compared to people not taking aspirin, but as bleeding was also rare, people would only be increasing their actual risk by 0.47%.

Under the headline "Bleedin' deadly," The Sun states: "New figures from a study by King's College London reveal up to one in 200 people treated with aspirin suffer a serious bleed".

The report neglected to point out that aspirin is beneficial for people who have had a heart attack or stroke already, until quite far into the story. This could panic people who need to take aspirin into stopping, putting them at unnecessary risk.

What kind of research was this?

This was a systematic review and meta-analysis. These are usually the best types of study to get an overall picture of the state of evidence on any particular topic.

What did the research involve?

Researchers searched for randomised clinical trials which included at least 1,000 people without cardiovascular disease, gave them either aspirin or no aspirin, and followed them up for at least a year.

They looked at the trial results for:

  • any combination of heart attack, stroke, or death from cardiovascular disease
  • any major bleed

They also looked separately at subgroups including people at higher and lower risk of a heart attack or stroke, assessed as having a 10% or above (higher) risk of having a heart attack or stroke in the next 10 years.

In addition, researchers looked for any differences in the numbers of people who were diagnosed with cancer or died from cancer, as there is conflicting evidence on the effect of aspirin on cancer risk.

What were the basic results?

As expected, aspirin reduced risk of heart attack and stroke:

  • 5.71 in every 1,000 people who had aspirin had a heart attack or stroke each year
  • 6.14 in every 1,000 people who did not have aspirin had a heart attack or stroke each year

That's a risk reduction of heart attack or stroke of 11% (hazard ratio (HR) 0.89, 95% credible interval (CrI) 0.84 to 0.95), but an absolute risk reduction of 0.38% (95% confidence interval (CI) 0.20 to 0.55).

Also as expected, aspirin increased risk of major bleeding:

  • 2.31 in every 1,000 people who had aspirin had a major bleed each year
  • 1.64 in every 1,000 people who did not have aspirin had a major bleed each year

That's a risk increase of major bleeding of 43% (HR 1.43, 95% CrI 1.30 to 1.56), but an absolute risk increase of 0.47% (95% CI 0.34 to 0.62).

Researchers found similar results when looking separately at people with higher or lower risk of cardiovascular disease. They found no effect of aspirin on cancer diagnoses or cancer deaths.

How did the researchers interpret the results?

The researchers said: "The current study demonstrates that when considering the totality of evidence, cardiovascular benefits associated with aspirin were modest and equally balanced by major bleeding events."

They added that it is difficult to compare the severity of heart attacks and strokes with major bleeding, but said "the absolute risk reduction for cardiovascular events and absolute risk increase for major bleeding associate with aspirin use were of similar magnitude."


This study adds evidence to confirm what doctors in the UK already knew – that aspirin increases the risk of bleeding and reduces the risk of heart attacks and strokes. The study adds useful information to make clear that people without cardiovascular disease benefit from only a small reduction in risk of heart attack or stroke, while increasing their risk of bleeding.

Because heart attacks are more common than major bleeding, the change to the absolute risk of either event with aspirin is about the same.

If you are concerned about your risk of a heart attack or stroke, talk to your doctor. You will get advice about your risk, and what you can do to reduce it. Find out more about cardiovascular disease risk.

If you are taking regular low-dose aspirin to prevent heart attack or stroke on the recommendation of a doctor, do not stop taking it without talking to your doctor. You can discuss your risk of bleeding, and whether it is outweighed by the reduced risk of a heart attack or stroke. Find out more about aspirin for prevention of a second heart attack or stroke.

NHS Attribution