THE QUESTION Millions of people take a daily aspirin, hoping it will help prevent heart problems. Might they be gaining some anti-cancer benefits as well?

THIS STUDY analyzed data from six reviews of multiple studies and four separate studies, involving thousands of participants who took 75 to 325 milligrams of aspirin daily starting when they were 50 to 65 years old. Compared with national rates for U.S. and U.K. residents, several digestive-system cancers were less common among those who took a daily aspirin for 10 years. Specifically, with daily aspirin use, colorectal cancer rates were about 35 percent lower than the national averages, and deaths from that cancer were about 40 percent lower. The occurrence of and deaths from esophageal and stomach cancer also were lower, both by about 35 to 50 percent. The rate of occurrence for digestive cancers began to go down only after people had taken aspirin for three years; death rates fell only after five years of aspirin use. The main side effect among long-term aspirin users was an increased risk for bleeding problems, primarily gastrointestinal bleeding and especially after age 70.

WHO MAY BE AFFECTED? People 50 and older. Doctors often suggest a low-dose aspirin (generally 81 mg) every day to reduce the risk for heart attacks and strokes caused by blood clots. Studies have shown that aspirin can lower risk for people who have survived a heart attack or stroke or who have cardiovascular disease, but whether daily aspirin prevents cardiovascular problems for other people is less certain. Some studies also have suggested that daily low-dose aspirin may lower risk for such cancers as breast, ovarian, prostate and skin, as well as digestive-system cancers, but medical experts generally do not recommend taking aspirin solely for cancer prevention because of the increased risk for internal bleeding.

CAVEATS The study did not determine the optimal dose of aspirin for cancer prevention. It also suggested but did not definitively determine that there may be an age at which the potential harm from taking aspirin daily would outweigh the possible benefits; bleeding problems among the participants, for instance, became much more common and serious after age 70, but cancer rates also spike at that age.

FIND THIS STUDY Aug. 5 online issue of Annals of Oncology (click on “Advance Access”), annonc.oxfordjournals.org.

LEARN MORE ABOUT aspirin at www.nlm.nih.gov. Learn about colorectal cancer at www.nihseniorhealth.gov.

The research described in Quick Study comes from credible, peer-reviewed journals. Nonetheless, conclusive evidence about a treatment's effectiveness is rarely found in a single study. Anyone considering changing or beginning treatment of any kind should consult with a physician.