Aspirin has many uses ranging from the prevention of heart attacks to relieving the symptoms of a headache. However, the negative aspect of aspirin use is its ability to cause gastrointestinal irritation and bleeding, especially in individuals who drink alcohol regularly.
Two common formulations of aspirin are immediate release (IR) and enteric coated (EC). In a recent study by Grosser et al., researchers wanted to determine if pharmacological resistance to aspirin was common among patients. The study consisted of 400 healthy volunteers between the ages 18-55, who were monitored for their response to either immediate release or coated aspirin. The study was broken down into three phases.
In the first phase, participants received 325 mg of aspirin IR or EC, and then their platelet reactivity was measured 4-8 hours later. Results showed that patients who took the IR formulation responded better to the medication because platelet adhesion decreased by about 60%. In contrast, 40% of those who took the EC formulation did not even respond to the aspirin 4 hours after taking it and 17% did not respond to it after 8 hours.
In the second phase of the study, which occurred after a washout period, participants who did not respond to the EC aspirin were again given this formulation to see if results would change. It was found that in the second time around, some participants responded to the aspirin, but 27 patients still did not.
In the third and last phase of the study, these 27 participants once again took an EC aspirin but at a lower dose of 81 mg; it was taken daily for a 1-week period. Among these participants, only 1 person did not respond to the aspirin, however, researchers believe it may have been due to a lack of adherence. Since for the most part participants eventually responded to the aspirin, the authors of the study concluded that a "pseudoresistance" to aspirin existed, rather than an actual resistance.
The authors explain that since the enteric-coated formulation has delayed and reduced absorption in comparison to the immediate release formulation of aspirin, it may appear that a person is resistant to the medication, but in reality it is just taking longer to work. Therefore, the authors conclude, "Pharmacological resistance to aspirin is rare."
Grosser T, Fries S, Lawson JA, Kapoor SC, Grant GR, FitzGerald GA. Drug resistance and pseudoresistance: an unintended consequence of enteric coating aspirin. Circulation. 2013;127:377-385. Abstract