Aspirin 'Resistance' May Actually Be Caused By Pill's Coating, Study Suggests

12/06/2012 03:06pm ET
CHICAGO - MAY 26: Aspirin products sit on the shelf at a Walgreens drug store May 26, 2004 in Chicago, Illinois. A recent study has linked frequent use of aspirin with a reduced risk of breast cancer in women. (Photo by Scott Olson/Getty Images)

As many as one third of people who take daily aspirin for heart-health reasons don't seem to reap the drug's blood clot-busting benefits, and a new study has identified a possible reason for why: the pill's coating.

Researchers from the Perelman School of Medicine at the University of Pennsylvania found that the coating on aspirin seems to delay the absorption of the drug by the body, thereby resulting in "pseudoresistance" to aspirin's effects. Coating is added to some aspirins to make it more stomach-friendly. Aspirin is often taken as a regular measure because it's been shown in past research to lower the risk of repeat heart attack.

The findings "question the value of coated, low-dose aspirin," study researcher Dr. Garret FitzGerald, M.D., director of the Institute for Translational Medicine and Therapeutics, said in a statement. "This product adds cost to treatment, without any clear benefit. Indeed, it may lead to the false diagnosis of aspirin resistance and the failure to provide patients with an effective therapy. Our results also call into question the value of using office tests to look for such resistance."

In the study, published in the journal Circulation and funded in part by Bayer Health Care, researchers had 400 study participants with an average age of 26 undergo blood and urine tests to see how many of them were resistant to aspirin. They took one dose of 325-milligram immediate release aspirin or enteric-coated aspirin.

None of the study participants were found to have true resistance to aspirin, but some of them did have pseudoresistance to aspirin because of the coating. However, no one seemed resistant to aspirin if it didn't have the coating.

"Variable absorption caused a high frequency of apparent resistance to a single dose of 325 mg enteric coated aspirin (up to 49 percent) but not to immediate release aspirin (0 percent)," researchers wrote in the study.

FitzGerald told the Wall Street Journal that 90 percent of aspirin sold for heart-health reasons have coating.

Bayer spokeswoman Anne Coiley said that the results of the study may not apply to the real world, though, ABC News reported.

"The authors' suggestion that use of enteric coated aspirin should be questioned, based on these study results, is of concern given the study population and methodology used, neither of which reflect real-world clinical use," Coiley said in a statement obtained by ABC News.

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