Anyone in the habit of taking a daily aspirin should know the risks involved, say the researchers behind a new study: while it reduces the risk of a heart attack or stroke, it raises the risk of severe internal bleeding.

In this particular study we're talking about adults without existing heart disease conditions, and scientists say the potential dangers outweigh the potential benefits – so think twice about popping an aspirin a day in the future.

The new research is a meta-study of previous clinical trials, looking at trends and patterns across more than 164,000 individuals, and it challenges conventional wisdom that daily aspirins are a safe way for cutting down the risk of heart disease, especially for older people.

"This study demonstrates that there is insufficient evidence to recommend routine aspirin use in the prevention of heart attacks, strokes and cardiovascular deaths in people without cardiovascular disease," says one of the researchers, Sean Zheng from King's College London in the UK.

"There has been more uncertainty surrounding what should be done in patients who are at higher risk of cardiovascular disease and in patients with diabetes. This study shows that while cardiovascular events may be reduced in these patients, these benefits are matched by an increased risk of major bleeding events."

Even before this study arrived, the experts said people should only take regular low doses of aspirin on a doctor's advice – there are a lot of health factors to consider with the blood-thinning drug, even though we know it can be a very effective medicine.

Now the picture is a little more clear. Over the thousands of people included in the meta-study, those who took daily aspirin had a 0.38 percent lower absolute risk of heart attacks, strokes, or death from cardiovascular events.

At the same time, a daily aspirin habit was associated with a 0.47 percent higher absolute risk of severe internal bleeding. That underlines how important it is not to start doing this until you've chatted to your doctor about it.

Kevin McConway from the Open University in the UK, who wasn't involved in the research, explained how that would translate into real figures to Rich Haridy at New Atlas.

With a daily aspirin, 57 rather than 61 people per 10,000 would be expected to suffer a heart attack or stroke; at the same time, 23 rather than 16 people per 10,000 would experience severe bleeding, on average.

"This seriously questions whether people who have not previously had heart attacks or strokes should be taking aspirin with the aim of reducing future cardiovascular events," Zheng told Lisa Rapaport at Reuters.

The median average age of all the study participants was 62 (ranging from 53 to 74). Around half of them were followed for at least five years.

One of the limitations of the research is that the multiple studies it looked at covered differing daily doses of aspirin – from 50 mg to 500 mg. Doctors don't normally prescribe anything higher than 100 mg a day in any case.

Even so it's worth patients and doctors weighing up the pros and cons of using aspirin as a preventative measure, and maybe considering alternative options (like controlling blood pressure or quitting smoking) instead.

In an editorial accompanying the study, Michael Gaziano from Brigham and Women's Hospital in Boston, who wasn't involved in the main research, says aspirin still remains "an important medication" for preventing cardiovascular health issues – as long as we use it wisely.

"Aspirin use requires discussion between the patient and their physician, with the knowledge that any small potential cardiovascular benefits are weighed up against the real risk of severe bleeding," says Zheng.

The research has been published in the Journal of the American Medical Association.