A new study has found that patients who are resistant to aspirin are four times more likely to have a heart attack, stroke, or even die from a pre-existing heart condition.
Some patients are prescribed aspirin over the long-term to cut their chance of developing a blood clot, reports BMJ.
Those who are labelled 'aspirin resistant' have blood cells (platelets) that are not affected in the same way as of those patients who are receptive to the drug i.e. 'aspirin sensitive.'
Presently, there is no agreed way of accurately checking who is and isn't aspirin resistant. However, the reason why some patients are aspirin resistant is controversial.
Few studies have looked at the impact of aspirin resistance on health outcomes for the patient.
Now, Canadian researchers have reviewed 20 studies looking at the effect. They involved 2,930 patients with cardiovascular disease, all of whom had been prescribed aspirin to prevent clots from forming in the blood.
More than a quarter (28 percent) of people were classified as aspirin resistant.
The researchers found that all aspirin resistant patients, regardless of their underlying clinical condition, were at greater risk of suffering a heart attack, stroke or even dying.
They found that 39 percent of aspirin resistant patients suffered some sort of cardiovascular event as compared to 16 percent of those that responded to the drug.
It was also found that taking other drugs to thin the blood, such as Clopidogrel or Tirofiban, failed to benefit those patients resistant to aspirin.
The researchers conclude that further studies on aspirin resistance are needed to identify the most useful test to determine the condition. They also say aspirin resistance: "is a biological entity that should be considered when recommending aspirin as antiplatelet therapy."
The study was published online in the British Medical Journal (BMJ).