Aspirin resistance is extremely rare


Aspirin resistance is extremely rare


Pharmacological resistance to aspirin is very rare, despite various estimates that have been put forward over the last few years, researchers from the University of Pennsylvania reported in the journal Circulation.

The authors say that the incidence of aspirin resistance is so low as to be considered nonexistent. They add that false diagnoses could be pushing up the numbers of people unnecessarily being placed on more expensive medications, with more side effects, to lower their risk of stroke and heart attack. If a patient is taken off aspirin to protect from cardiovascular events, they are usually prescribed a blood thinner, such as Plavix, which can make them bleed and bruise more easily.

Low dose aspirin is known to reduce the risk of heart attack and stroke, the authors explained as background information. Experts have said that aspirin resistance may undermine aspirin therapy. However, even though there has been concern among health care professionals and patients regarding aspirin resistance, the term has no clear definition.

The researchers wrote that estimates regarding how common or rare aspirin resistance is have varied considerably (most doctors mistakenly believe that about one third of patients experience aspirin resistance). They explained that they "aimed to determine the commonality of a mechanistically consistent, stable and specific phenotype of true pharmacological resistance to aspirin - such as might be explained by genetic causes."

Lead researcher, FitzGerald and team recruited 400 healthy volunteers and screened their responses to a single oral immediate release or enteric coated aspirin, with a dose of 325 mg. Participants who had appeared aspirin resistant once were tested again; if they were still resistant they were to be exposed to 81 mg low dose enteric coated aspirin and 75 mg clopidogrel for one week each.

They found that absorption rates influence people's likelihood to apparent aspirin resistance - up to 49% of those on a single 325 mg enteric coated aspirin dose had apparent resistance, while none had any resistance when on the immediate release aspirin.

In other words, the scientists found that the coatings added onto the surface of aspirin tablets mask the uptake of the medication in the blood, leading to conclusions that the aspirin is not working. They are then put onto a prescription blood thinner, which is much more expensive.

Approximately 20% of adults in the USA take low-dose aspirin daily to reduce their risk of heart attack and stroke. The majority of patients take the coated versions, which are marketed as being easier on the stomach. FitzGerald and team stress that there is no evidence showing that coated aspirins are more tolerable than the immediate release ones.

Patients and doctors should not worry about resistance, FitzGerald emphasized. Those taking aspirin should consider buying the cheaper immediate release versions (uncoated ones).

The scientists concluded:

"Pharmacological resistance to aspirin is rare; this study failed to identify a single case of true drug resistance. Pseudoresistance, reflecting delayed and reduced drug absorption, complicates enteric coated but not immediate release aspirin administration."

German pharmaceutical company Bayer, a major global seller of aspirin, said that the study had one flaw - all the 400 participants were relatively young and healthy, rather than sick patients who would be taking aspirin to protect against cardiovascular events.

Coated versions of aspirin cost about 6 cents per tablet in the USA, compared to just 1 cent for the immediate release ones.

The team also wonder how valuable aspirin-resistant blood and urine tests are.

Recent research on aspirin

Over the last few years, there have been many studies on aspirin. A painkiller that has been on the market for over 100 years, and which seems to have a number of therapeutic uses, apart from reducing inflammation and pain. Below are examples of some studies:
  • Aspirin appears to reduce the risk of liver cancer, as well as improving the outlook for patients with liver disease
  • Aspirin may protect women from developing ovarian cancer
  • Aspirin may slow down brain decline in elderly females
  • Aspirin may help prostate cancer patients live longer
  • Heart surgery patients who are given aspirin have a lower risk of renal failure
  • Aspirin may reduce the chances of developing skin cancer


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Section Issues On Medicine: Cardiology