Anti-Clotting Effect of Aspirin Wanes Over Time
(NY, Reuters Health, 3/25/04)

When Dr. Fabio M. Pulcinelli and his associates at the University "La Sapienza" in Rome, screened blood samples from 150 patients who were undergoing aspirin therapy to prevent blood platelets from clumping into clots, the researchers found that, after 2 months of treatment, the maximal percentage of platelets that could be made to clump fell from 88% to 38%, but then the aspirin apparently lost its effectiveness as clumping rose to 46% at 6 months, 48% at one year and 62% at two years. In contrast, a group of 80 comparable patients did not experience a decline in the anti-coagulation action of the drug, ticlopidine, during a two-year period. These findings highlight the possibility that prolonged use may desensitize patients to aspirin’s anti-coagulation effect.