Low dose aspirin did not prevent cancer in healthy women.

M e t h o d s Design: Randomized placebo-controlled trial (Women’s Health Study [WHS], a randomized 2 × 2 factorial trial). Allocation: Concealed.* Blinding: Blinded {participants, health care providers, data collectors, and outcome assessors}†.* Follow-up period: Mean 10.1 years. Setting: {A mail-based trial in female health care professionals in the United States}†. Participants: 39 876 women ≥ 45 years of age (mean age 55 y) without a history of cancer (except nonmelanoma skin cancer), cardiovascular disease, or other major chronic illness and no history of adverse effects to aspirin who were not taking aspirin or nonsteroidal antiinflammatory drugs > once per week; anticoagulants or corticosteroids; or individual supplements of vitamin A, vitamin E, or β-carotene > once per week. Intervention: Aspirin (100 mg every other d) (n = 19 934) or matching placebo (n = 19 942). Outcomes: Total invasive cancer, excluding nonmelanoma skin cancer. Secondary outcomes were breast, colorectal, and lung cancer. Patient follow-up: 97% for morbidity and 99% for mortality (intention-to-treat analysis).