Previous studies have also suggested this may be the case.
However, to achieve the benefit it appears that more than a decade of use, at the strongest dose levels, is required, and that in itself carries the risk of side effects such as bleeding.
Similar results were also found for non-steroidal anti-inflammatory drugs (NSAIDS) such as ibuprofen and naproxen.
The report, from researchers at Massachusetts General Hospital (MGH), Brigham and Women's Hospital (BWH) and Dana-Farber Cancer Institute, acknowledges that earlier studies have found that, among patients with a history of colon polyps or cancer, regular aspirin treatment prevents the recurrence of precancerous polyps.
But it also points out that the ability of aspirin to reduce the long-term incidence of invasive cancer has not been well-demonstrated.
Andrew Chan, MD, MPH, of the MGH Gastrointestinal Unit, and the paper's lead author, says their study did find a protective effect of long-term aspirin use on risk of invasive colorectal cancer, but only at dosage levels considerably higher than those used to prevent cardiovascular disease.
The Nurses' Health Study has followed more than 120,000 female registered nurses since the mid-1970s, asking them to complete a questionnaire on risk factors for, and incidence of, cancer and cardiovascular disease, every two years.
Assessments of diet and the use of aspirin and NSAIDS were added to the questionnaire in 1980, and the current report analyzes information from almost 83,000 NHS participants, among which 962 cases of colorectal cancer were diagnosed during the 20-year study period.
The researchers found that while the incidence of colorectal cancer was lower in the women who took aspirin regularly, the risk reduction was significant, only for those taking aspirin 10 years or longer, and the benefit increased as dosage levels rose, with the greatest risk reduction seen in those taking more than 14 standard tablets per week.
According to the researchers, similar risk reduction was seen with the intake of NSAIDS, with greater benefit also associated with higher dosage; but an analysis of acetaminophen, which is believed to act through different mechanisms, found no association of that medication with colorectal cancer.
It is believed that the ability of aspirin and NSAIDS to reduce cancer risk may, at least in part, relate to their shared ability to inactivate the COX-2 enzyme, which could stimulate tumor development.
The risk of serious gastrointestinal bleeding is a known side effect of both aspirin and NSAIDS, and that risk increased as dosage levels increased, with bleeding occurring nearly twice as often in those taking the highest doses.
The researchers have estimated that a high-dose aspirin regimen, that prevented one or two cases of colorectal cancer in a population, might also contribute to eight additional cases of serious gastrointestinal bleeding.
Chan says that before any recommendations about whether patients should take these medications to reduce their cancer risk are made, additional studies need to be done to clarify the risks and benefits of such an approach, compared to other preventative strategies.
Chan, who is an instructor in medicine at Harvard Medical School, says at present individuals need to discuss the options with their physicians.
Studies aimed at clarifying the impact of long-term use of aspirin and NSAID drugs, particularly in those at high risk for cancer and other chronic diseases, have apparently, already been initiated by Chan and his colleagues.
The Nurses' Health Study was initiated in 1976 at BWH, and is the longest-running, major women's health study ever undertaken.
It has resulted in hundreds of journal articles, many containing groundbreaking findings on how to prevent some of the major causes of disease and death in women.
The report appears in the August 24 JAMA: The Journal of the American Medical Association.