Patients resistant to aspirin may have an increased risk of heart attacks or strokes after taking the medication, new research suggests.
Scientists at Toronto General Hospital and Ontario in Canada looked at 20 studies concerning people who were told to take aspirin long-term to reduce the risk of blood clots.
These involved 2,930 patients with cardiovascular disease, and 28% were classified as aspirin resistant.
Of these, 39% were found to be more likely to suffer a heart attack, stroke, or even die, compared with 16% of people that responded to the drug.
The study is published online in the British Medical Journal (BMJ).
The authors concluded: "Meta-analysis showed that patients who are 'resistant' to aspirin are at greater risk of clinically important adverse cardiovascular events, regardless of the assay used to measure aspirin resistance.
"Not only did aspirin resistance have an effect on clinical outcome but this risk was not ameliorated by currently used adjunct antiplatelet therapies.
"Patients who were classified as aspirin resistant were at about a fourfold increased risk of non-fatal and fatal cardiovascular, cerebrovascular, or vascular events while taking aspirin than their aspirin sensitive counterparts.
"This risk can be generalised to a wide variety of patient populations with cardiovascular or cerebrovascular disease."