This new research shows that low-dose aspirin may serve as a protection against colorectal cancer. The study was based on a long-term trial in a large group of women, who have been underrepresented in studies on this topic, said lead author Nancy R. Cook, associate biostatistician at Brigham and Women's Hospital and professor at Harvard Medical School. The study is being published this week in the journal Annals of Internal Medicine.
This builds on a significant body of research showing the potential preventative properties of aspirin in many cancers such as melanoma. This study did not find a strong link between aspirin and preventing any other kinds of cancer during the 18-year period, however.
None of this means that aspirin is right for everyone. Always consult a doctor before starting a new medical routine.
The study included nearly 40,000 women 45 and older who were part of the Women's Health Study. About 34,000 participated in follow-up. Researchers followed the women for up to 18 years from the start of the study.
The study participants were randomly assigned to either take 100 milligrams of aspirin or a placebo every other day. The alternate-day strategy was intended to reduce the side effects of aspirin, Cook said.
Some participants also took vitamin E on the nonaspirin days; the others took a placebo.
In the first 10 years, the researchers did not find an association between aspirin use and colorectal cancer; there seemed to be no benefit over taking a placebo.
But in women who were followed for longer than 10 years, the benefits of taking aspirin became clear: Between 10 to 18 years after the start of the study, researchers observed a 42% reduction in colorectal cancer in the group that was taking aspirin, relative to the women who were not. That averages out to about a 20% reduction over the length of the study.
Researchers did not find that age or other factors were significantly associated with the study results. Socioeconomic status was balanced between the group taking aspirin and those who received placebos.
Other studies have also found a delayed effect in aspirin's preventative properties, Cook said. Cancer usually takes about 10 years to develop from its early stages. It's possible that aspirin delays the formation of polyps, she said. Some studies have suggested that higher doses of aspirin have a shorter latent period - in other words, less time passes before the preventative benefits are seen.
On the downside, women taking aspirin were more likely to develop gastrointestinal bleeding and peptic ulcers, based on the information that women reported about themselves.
The researchers did not find any cardiovascular benefit to taking aspirin every other day, contrary to the preventative properties of aspirin against heart attack and stroke that have been found in many other studies, especially in men.
"We're not quite sure if it was because it was the low dose or because it's different in women than men," Cook said.
Similarly, although other studies have found that aspirin may prevent other kinds of cancers, this study did not find associations with any other kind of cancer. This could also be because of the low dose, but more research would need to be done to confirm.
No benefit to the vitamin E, taken every other day, was observed.
This study adds to a large body of research that already established the potential of aspirin to prevent colon cancer, which had mostly been shown in men, Cook said. This study highlights the drug's protective potential in women.
Those with an elevated risk of the disease - for instance, a family history of colorectal cancer, or frequent polyps - should consider aspirin, Cook said.
Cook doesn't advocate taking aspirin every other day; daily is easier to remember. But consider the possible downsides as well. Aspirin increases the risk of bleeds; in rare cases, it can promote hemorrhagic stroke or gastrointestinal hemorrhage, Cook said. More commonly, peptic ulcer or gastric upset may occur.
In an accompanying editorial, Dr. Peter Rothwell of Oxford University noted the findings reinforce the idea that risks and benefits of aspirin should be considered separately in men and women.
"People really need to balance risks and benefits, and discuss with doctor whether they should be taking it," Cook said.