Aspirin should not routinely be used to prevent heart attacks in people with diabetes, Scottish research suggests.
The British Medical Journal reported that in 1,300 adults with no symptoms of heart disease the drug, which can cause stomach bleeds, had no benefit.
The findings contradict many guidelines which advocate people with diabetes use aspirin to counter the underlying high risk of heart attack and stroke.
But there are key high-risk groups who still need the drug, experts said.
In people who have already had a heart attack or stroke, or have been diagnosed with coronary artery disease, aspirin has been shown to reduce the risk of future "events" by around 25%.
However, in recent years doctors have begun to focus on people who have not yet developed so-called cardiovascular disease, but are at high-risk of having it in the future - such as people with diabetes.
There are around two million people over 40 with diabetes in the UK.
Around 80% of people with diabetes die of cardiovascular disease including strokes and heart attacks.
A daily dose of aspirin is recommended by several UK guidelines as a "preventive" treatment in these groups.
But in the latest study in adults over 40 years with type 1 or type 2 diabetes and no symptoms of cardiovascular disease, there was no difference over seven years in heart attacks or strokes between those given aspirin and those given a dummy pill.
Study leader Professor Jill Belch, from the University of Dundee, said aspirin was one of the most common causes of hospital admission for gastrointestinal bleeding.
"We have got a bit ahead of ourselves with aspirin.
"We need to think again about using it for primary prevention."
However she stressed the drug was beneficial in people who had already had a heart attack or stroke.
Professor Peter Sever, an expert in clinical pharmacology and therapeutics at Imperial College London, said the study was "extremely important".
"It confirms many concerns we have that aspirin is very widely used in the general population without an evidence base to support its overall benefits.
"Thousands of people buy aspirin over the counter - I'm forever saying to patients you shouldn't be taking this.
"I have had a couple of patients admitted to hospital with major gastrointestinal bleeding when there was no evidence it was doing any good."
The number of people diagnosed with diabetes and as having a high risk of cardiovascular disease is set to increase, with government plans in England to introduce a national screening programme for the over-40s next year.
Professor Steve Field, chair of the Royal College of GPs, said it would be worth revisiting the guidelines.
"But patients shouldn't panic or stop taking aspirin," he said.
Judy O'Sullivan, cardiac nurse at the British Heart Foundation, said: "This study adds weight to the evidence that aspirin should not be prescribed to prevent disease of the heart and circulation to people with diabetes, and other high risk groups, who do not already have symptoms of the disease."