CHICAGO – Men who took aspirin over five years slightly lowered their risk for prostate cancer, but women who took low doses over 10 years didn’t reduce their risk of cancer, two separate studies indicate.

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, the director of cancer prevention at the National Cancer Institute – which helped fund the Women’s Health Study – Dr. Peter Greenwald, said.

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% 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, the lead researcher for the Women’s Health Study, Dr. Julie Buring of Harvard’s Brigham and Women’s Hospital, said.

But she said her study can’t rule out whether higher aspirin doses taken daily would protect women against cancer, too.

The women’s study appears in today’s Journal of the American Medical Association. The men’s study is in today’s Journal of the National Cancer Institute.

Researchers have theorized that aspirin’s anti-inflammatory effects might protect against cancer in which inflammation is thought to play a role, chiefly colorectal cancer but potentially also prostate and breast cancers, said American Cancer Society epidemiologist Eric Jacobs, lead author of the aspirin prostate cancer study.

Still, Mr. Jacobs noted, “The American Cancer Society doesn’t recommend that men or women start taking aspirin or any other NSAIDS to prevent cancer.” They can cause serious side effects, and there’s inadequate evidence about potential cancer-fighting benefits, the cancer society says.

Earlier this year results from the same women’s study suggested that low-dose aspirin every other day reduced the risk of strokes, but not heart attacks – findings opposite of what has been found in men. Dr. Buring said the researchers chose to study the lowest possible aspirin dose to reduce risks of side effects.

The women’s study also looked at vitamin E and similarly found no protection against cancer, echoing previous studies. There was a 24% reduction in cardiovascular deaths among women who took vitamin E, but researchers said they couldn’t explain the findings and believe more study is needed. They said the results weren’t strong enough to recommend taking vitamin E pills to avoid dying from heart disease.