Prophylactic aspirin often stops working

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Can a person who takes aspirin daily to ward off a stroke or MI build up a resistance to the drug, undermining its ability to block platelet aggregation?

Studies have yielded inconsistent answers to this important question, but a British research team told the recent International Stroke Conference in San Francisco that it had arrived at a definite answer: Aspirin resistance is real—and occurs fairly often.

The researchers analyzed data from 19 trials that enrolled a total of 2,696 patients who took aspirin prophylactically for either cardiovascular or cerebrovascular disease. Dosages ranged from 75 mg a day to 500 mg three times a day, and resistance was measured by platelet aggregometry or serum assays.

Results of the meta-analysis demonstrated that 758 patients, or 26%, developed aspirin resistance, which left them vulnerable to severe disease: 41% suffered either a stroke, MI, unstable angina, or a graft failure compared with 17% of those who remained aspirin-sensitive.

“Clinicians should screen for aspirin resistance,” particularly in patients with renal impairment, said lead investigator George Krasopoulos, MD, PhD, senior registrar at Royal Brompton Hospital in London. “If you have it, you're at six times greater risk of dying and four times greater risk of developing any cardiovascular event. You also have four times the risk of an MI or unstable angina.” If a patient is found aspirin-resistant, then clinicians should prescribe another anti-platelet medication, he added.

Meanwhile, if any of your patients take aspirin to prevent colon cancer, tell them to stop, said the U.S. Preventive Services Task Force. After surveying the literature, the panel concluded that the risks of taking ≥300 mg of aspirin daily outweigh the benefits, and lower dosages don't confer any protection against colorectal cancer (Ann Intern Med. 2007;146:361-364).

The new recommendation applies to asymptomatic adults at average risk for colorectal cancer, including those with a family history of the disease, but not to those with hereditary nonpolyposis colon cancer syndromes, familial adenomatous polyposis, or a history of colorectal cancer or adenomas. According to the panel members, clinicians should continue to discuss aspirin chemoprophylaxis with patients who are at increased risk for coronary heart disease.

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