By increasing the activity of platelets, blood cells that help clot the blood, smoking may weaken the anti-clotting effect of aspirin, new research suggests.

A small daily dose of aspirin is usually recommended for patients who have had a heart attack or stroke to prevent platelets from building up and blocking critical blood vessels.

“Aspirin and other antiplatelet treatments are central to the prevention of heart attack and stroke,” Dr. Michael Domanski told Reuters Health. “A clearer understanding of how to best predict the level of protection provided in a specific individual is a research question of potentially great public health importance.”

As they report in the American Journal of Cardiology, Domanski of the National Heart, Lung and Blood Institute in Bethesda, Maryland and colleagues conducted a study to determine which factors predict a poor anti-clotting response to aspirin.

About one third of the 123 patients in the study were smokers. Their ages ranged from 21 to 95 years. About half of the subjects were women and most had high blood pressure, as well as heart disease.

Sixty-six patients were taking low-dose aspirin at 81 milligrams per day, and tests showed that eight of them did not respond to the drug. When the dose was increased to 325 mg per day, only one remained resistant.

In the other 57 patients, who were taking 325 mg of aspirin per day, three were resistant. When they were given a dose of clopidogrel (Plavix), another anti-platelet drug, and tested 4 hours later, two had become responders.

In the final analysis, smokers were nearly 12-times more likely to be resistant to aspirin than were non-smokers. Thus, the researchers conclude, the finding “adds still more weight to the importance of abstinence from smoking.”

SOURCE: American Journal of Cardiology, September 2006.