The study's findings don't necessarily rule out that a larger, daily aspirin dose protects against cancer. A standard aspirin tablet contains 325 milligrams, while a so-called baby aspirin, taken to protect against heart attack or stroke, contains 81 milligrams.

The much-anticipated findings, published in the Journal of the American Medical Association, are from the Women's Health Study, the largest and longest trial to compare aspirin or vitamin E to a sugar pill, or placebo. They contradict some earlier, less rigorous studies.

For example, some previous studies suggested that aspirin protects against breast and colon cancer, but the Women's Health Study did not find that to be the case. It found only a "marginally statistically significant" reduction in lung cancer cases and deaths among aspirin users. And some suggested that vitamin E's antioxidant properties could reduce heart disease and cancer risk.

Although the new vitamin E data do not show any cancer benefit, the authors report finding about a 25% reduction in heart attacks, strokes and cardiovascular deaths in women 65 years and older. In March, Women's Health Study scientists reported that vitamin E was safe but provided no cardiovascular benefit to study participants as a whole.

Low-Dose Aspirin

Low-dose aspirin reduced women's risk of strokes but not heart attacks, researchers also reported in March. Aspirin protected against heart attacks only in those 65 and older.

The Women's Health Study involved 39,876 apparently healthy women 45 years and older. Each was randomly assigned to take 100 milligrams of aspirin or a placebo and 600 International Units of vitamin E or a placebo. They took the pills every other day for 10 years.

Julie Buring of Boston's Brigham and Women's Hospital, senior author of the vitamin E and aspirin papers, says only one other randomized trial of the supplement in women has broken down its results by age, and it did not find a cardiovascular benefit in older participants.

Buring says she's asking others who've conducted similar trials to analyze their findings by age. "You always need more than one study to know," she says.

Larger Aspirin Dose?

The one benefit found in the aspirin/cancer analysis, fewer lung cancer cases and deaths, could have been a result of chance, says lead author Nancy Cook of Brigham and Women's Hospital.

Still, Cook says, the study's findings don't necessarily rule out that a larger, daily aspirin dose protects against cancer. A standard aspirin tablet contains 325 milligrams, while a so-called baby aspirin, taken to protect against heart attack or stroke, contains 81 milligrams.

Charles Fuchs of Boston's Dana-Farber Cancer Institute called the new study "very well performed." But three other trials comparing 81 milligrams to 325 milligrams of aspirin a day to a placebo all found a reduced risk of pre-cancerous colon polyps, Fuchs said, adding that another lengthy trial randomly assigning women to aspirin or a placebo is needed to clarify the role of a larger dose in cancer prevention.

Meanwhile, many methods proven to reduce cancer risk, such as quitting smoking or colon cancer screening, are underused, says American Cancer Society epidemiologist Eric Jacobs, co-author of an accompanying editorial.

Jacobs co-authored a study out today in the Journal of the National Cancer Institute that found men who regularly used aspirin or other anti-inflammatory pain-relievers for five years were 18% less likely to develop prostate cancer than men who didn't use such drugs.