(Bloomberg) — There’s no such thing as aspirin resistance, according to a study that suggests a false diagnosis may be unnecessarily raising the number of people given costlier prescription drugs with more side effects to lower their risk of heart attack and stroke.

The report, by University of Pennsylvania researchers, finds the coatings put on aspirin by makers such as Bayer AG mask uptake of the medicine in the blood, leading to false diagnoses that it’s not working, said the study published Tuesday in the journal Circulation. That’s when patients are put on a prescription treatment, such as Bristol-Myers Squibb Co.’s Plavix blood thinner.

One in five Americans take a low-dose aspirin tablet daily to reduce the risk of heart problems, with most taking coated versions sold as less harsh on the stomach, the university said in a statement. Garret FitzGerald, a lead researcher on the study, said there’s no scientific evidence yet showing the coatings make aspirin more tolerable.

“The takeaway for consumers is, don’t worry about resistance,” FitzGerald says. “And if you’re taking aspirin, there’s plenty of reason to take the cheap, immediate-release uncoated version.”

Doctors had estimated aspirin may not work for about a third of patients, FitzGerald says.

Bayer, which contributed funding for the research, says a delayed uptake of coated aspirin “would not be unexpected.” The Leverkusen, Germany-based drugmaker’s statement says the study was flawed because it tested aspirin on a relatively young, healthy population instead of the sick patients who would typically take it to prevent heart problems.

FitzGerald says he didn’t question the effectiveness of coated tablets, but saw no evidence they provided any more value for their price. There’s no reason to suggest sick patients would have a genetic resistance to aspirin while healthy patients wouldn’t, he says.

While the research suggests Bayer’s higher-cost coatings may do more harm than good, it also contained some positives for the company since it found more people may benefit from use of aspirin as a blood thinner than previously thought, says FitzGerald, director of the Institute for Translational Medicine and Therapeutics at the Philadelphia school.

The research also calls into question the value of blood and urine tests sold to diagnose supposed resistance, FitzGerald says. Coated versions of aspirin, meanwhile, may cost 6 cents a tablet compared to less than one cent for bare pills, he says.

“That’s unlikely to be a deal-breaker for most people, but if you look at it from the level of national health spending, and in an era of cost constraints, it starts to add up,” FitzGerald says.

The study gave 400 patients coated and uncoated versions of the medicine and found no consistent pattern of resistance, researchers say in the journal, which is published by the Dallas-based American Heart Association.

Plavix can have side effects including making patients bleed and bruise more easily, New York-based Bristol-Myers says on its website.

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