When Aspirin Doesn't Work

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It seems we are always learning something new and unexpected about the medications we take, and last week the surprise for me was about aspirin. A lot of people, it turns out, are born with--or are developing--a resistance to aspirin. This is not the kind of resistance that drug users build up over time or that bacteria develop against antibiotics, but it is nonetheless important. Aspirin is one of our most commonly used over-the-counter medications and is something of a miracle drug. It can do everything from dull the pain of a headache to reduce--as much as 25%--the risk of a stroke or heart attack.

So it was somewhat unsettling to read in Circulation, the journal of the American Heart Association, that about 30% of Americans are aspirin resistant, which means they have to either take more aspirin to get the same effect or use a different drug. To find out more about the study, I called Dr. Salim Yusuf, one of its authors and a professor of cardiology at McMaster University. "This very well-known drug may have serious limitations," he told me. "And those people who are aspirin resistant have a higher risk of dying from heart disease than those who are not."

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When aspirin is doing its job, it works by thinning the blood and blocking a chemical called thromboxane, which promotes the formation of life-threatening clots in the arteries. In some people, however, aspirin does not adequately block thromboxane, and it is this phenomenon that scientists are calling aspirin resistance.

Dr. Yusuf and his colleagues discovered it by measuring thromboxane levels in the urine of more than 5,500 regular aspirin users. "It became clear that the aspirin was not affecting everyone equally," Yusuf said. The difference was significant. Patients who had high levels of thromboxane in their urine had a risk of cardiovascular-related death that was 3 1/2 times as great as those with low levels.

Before you rush out and have your urine tested, however, you should know that nobody is quite sure what the ideal urinary thromboxane level is. That makes routine testing difficult. Moreover, Yusuf was quick to add, the study doesn't suggest that anybody should stop taking aspirin. What it suggests is that some people may need more protection than aspirin alone can offer. Doctors I consulted said they would not recommend changing medications on the basis of this study but they might be quicker to put high-risk patients on more powerful anticlotting drugs, such as Plavix or Coumadin. For the rest of us, "Take two aspirin and call me in the morning" probably still applies.

Dr. Gupta is a neurosurgeon and CNN medical correspondent