(HealthDay)—A daily, low dose of aspirin may increase the risk for progression and metastasis as well as mortality related to later-stage disease in adults aged 65 years and older who develop cancer, according to a study recently published in the Journal of the National Cancer Institute.

John J. McNeil, M.B.B.S., Ph.D., of Monash University in Melbourne, Australia, and colleagues studied the daily use of 100 mg of in 16,703 Australians (aged 70 years and older) and 2,411 U.S. participants (aged 65 years and older) for a median of 4.7 years in a randomized, placebo-controlled, double-blind trial. All participants were free of , dementia, or cardiovascular disease. Cancer incidence and mortality were assessed.

A total of 981 cancer events occurred in the aspirin group, while 952 occurred within the placebo group. The researchers observed an association between daily aspirin intake and the occurrence of metastasis (hazard ratio [HR], 1.19; 95 percent confidence interval [CI], 1.00 to 1.43) or progression to a stage 4 diagnosis (HR, 1.22; 95 percent CI, 1.02 to 1.45) in patients with incident cancers. Additionally, patients were found to have an for death if their presenting cancer stage was at 3 (HR, 2.11; 95 percent CI, 1.03 to 4.33) or 4 (HR, 1.31; 95 percent CI, 1.04 to 1.64). No significant differences were observed in the occurrence of all incident cancers (HR, 1.04; 95 percent CI, 0.95 to 1.14), hematological (HR, 0.98; 95 percent CI, 0.73 to 1.30), or all solid cancers (HR, 1.05; 95 percent CI, 0.95 to 1.15).

"Possible explanations for this finding include aspirin suppressing (or 'blunting') antitumor inflammatory or immune responses critical to controlling later-stage growth and spread," the authors write. "Such an effect may be particularly evident among an older population for which underlying antitumor immunity may already be compromised."

Several authors disclosed financial ties to Bayer.