ASPIRIN may fail to prevent a repeat heart attack in as many as one in five people, according to research carried out in eight Irish hospitals.
The drug is known to reduce the risk of clots forming and cut the chances of a stroke or heart attack.
However, research on patients who have already suffered a heart attack – and are at risk of another – has found aspirin does not protect everyone.
The study, led by Prof Dermot Kenny of the Royal College of Surgeons, found that those patients least likely to benefit from aspirin therapy are younger men in their 40s who have diabetes and high blood pressure.
They may also be obese and drink large amounts of alcohol, the Irish Heart Foundation's National Cardiovascular and Stroke Research Network found.
Prof Kenny said: "The patients at high-risk of recurrent heart attack are not getting the benefit of a cheap and generally effective therapy. Our study shows about 20pc of patients are not protected by their existing therapy."
Irish Heart Foundation medical director Dr Angie Brown said: "The findings of this study are significant and point to the critical role of clinical research in identifying this at-risk group of patients.
" We hope it will drive the establishment of new approaches to allow for accurate identification and management of this at-risk cohort of patients in secondary prevention."
Around 80,000 adults are diagnosed every year with coronary heart disease, and the number is due to rise to 103,000 by 2020.
More than 5,500 people were discharged from hospital in 2011 after suffering a heart attack, and more than 2,100 people died of heart attacks.
Low dose aspirin – around 75mg a day – should not be taken by people who have certain conditions such as peptic ulcer. It should also be used with caution by anyone who has asthma or uncontrolled blood pressure.