The conflicting results don't help settle the debate about whether aspirin and similar anti-inflammatory medicines could be used to prevent cancer. Doctors familiar with the research think different study designs and aspirin doses explain the contrasting findings.
"I don't think we have a final story on aspirin" and its effects on cancer, said Dr. Peter Greenwald, director of cancer prevention at the National Cancer Institute, which helped fund the Women's Health Study.
That study, involving nearly 40,000 women, is among the longest aspirin-cancer studies to date and used doses a little higher than in baby aspirin, taken every other day and compared against dummy pills. It found no effect on lymphoma, colorectal, breast or several other cancers, although results for lung cancer were less conclusive.
Those results contradict several smaller, less rigorous studies that in many cases used higher, more frequent doses.
In the men's study, American Cancer Society researchers followed 70,144 men over nine years and asked about their use of aspirin and other nonsteroidal anti-inflammatory drugs, or NSAIDS, including ibuprofen, such as Advil and Motrin. Men who took standard 325-milligram doses of those medicines daily for at least five years were about 18 percent less likely to get prostate cancer than men who used aspirin occasionally or for a shorter duration.
That kind of observational study can't rule out that men who decided to take aspirin were generally healthier and less likely to get cancer to begin with, said Dr. Julie Buring of Harvard's Brigham and Women's Hospital, lead researcher for the Women's Health Study.