Can daily use of aspirin reduce the risk of certain cancers?  Maybe it can.
(June 2007)

Low dose aspirin, for example 100 milligrams every other day, is recommended for prevention of heart attack In those who have documented coronary heart disease, have had a heart attack, or suffer from unstable angina (heart pain) or certain abnormal heart rhythms.  Its use is controversial in adults without known heart disease.  Healthful Life does not recommend it for ostensibly healthy adults; we believe that, in those without a history of coronary heart disease or stroke, any benefits are balanced by the risk of aspirin-induced bleeding into the intestinal tract or even into the brain.  The role of aspirin in the prevention of cancer is also unsettled.  A new study from the American Cancer Society provides some interesting data.  They enrolled more than 145,000 men and women, almost all over 50 years of age, in 1992-1993 and followed them for about ten years; during that time, 10,931 men and 7,196 women were diagnosed with cancer.  The enrollees were divided into four groups: no aspirin use reported; less than daily use; daily use for less than five years; daily use for more than five years.  Most of the daily users took one or two adult strength aspirin tablets each day ((325 milligrams per tablet).

Long-term daily aspirin use (more than five years) was associated with a 14 to 16 percent reduced risk of cancer occurrence in men and in women, but the reduction was statistically significant only in men.  This modest reduction in occurrence was attributed to two cancers in men and two in women.  For men, it was bowel cancer (32 percent reduction) and prostate cancer (19 percent reduction).  For women, it was bowel cancer (55 percent reduction) and breast cancer (17 percent reduction), but the reduced occurrence of breast cancer was not statistically significant.  There was no reduced risk for those who, at enrollment, had used aspirin for less than five years.

The authors were properly cautious in regard to the interpretation and application of their findings.  They noted “our results suggest that even long-term daily aspirin use is likely to be associated with relatively small, although potentially relevant, reduction in cancer risk.  Our results do not have immediate clinical implications.  Confirmation from randomized trials is necessary before a reduction in cancer risk could be considered a benefit of using adult-strength aspirin.  However, if daily adult-strength aspirin use is ultimately found to meaningfully reduce overall cancer risk, there could be important clinical implications with respect to who should be taking aspirin and at what dose.”

Commentary: this is a good and useful study.  Here are the issues and questions:

- This is an observational study in which those who decided to take aspirin were compared with those who did not.  Such studies are always subject to biases and confounders the investigators may not have taken into account.  For example, in this study, there was no evaluation of frequency of screening for bowel cancer or how much vigorous exercise the participants engaged in each week.  If aspirin users had more effective bowel cancer screening or engaged in more vigorous physical activity, that could have reduced bowel cancer occurrence rather than the aspirin use.

- As the investigators note, a definitive answer probably requires a long-term study in which some people are assigned to aspirin, others to an inactive placebo, following both groups for occurrence of and deaths from cancer.

- The dosage issue is unsettled.  Here, the dose used is not low dose, but rather one or more adult-strength aspirins a day.  That raises the obvious issue of benefits versus harms (bleeding into the intestinal tract or the brain).  No data were collected on adverse effects of daily aspirin use at this dosage.  No judgement can be made until such data are available.

- In addition to occurrence of new cases, we need to know whether aspirin use is associated with reduction in deaths from bowel, prostate, or breast cancer.  It may be that aspirin is mainly reducing risk of cancers that will not progress or cause death.  This is especially a concern with prostate cancer.

Jacobs, E.J., et al.  A large cohort study of long-term daily use of adult-strength aspirin and cancer incidence.  Journal of the National Cancer Institute.  Vol 99 (April 18) Pgs 608-615.  2007.

The data look best for bowel cancer, but, even here, the studies are contradictory; most do show a benefit in reduction of risk, but the negative studies include good prospective studies. 

This is a good study, especially in regard to bowel cancer.  An excellent review article on the effects of aspirin, at least 300 milligrams a day, on bowel cancer occurrence appeared in the May 12, 2007 issue of The Lancet.  That review supports this article, finding a similar benefit for those taking aspirin for at least five years; the reduction in bowel cancer occurrence was not found until at least ten years after initiation of aspirin treatment.  The review differed from this study in finding no reduction in the risk for developing either prostate or breast cancers.

Neither study analyzed the role of bowel cancer screening with sigmoidoscopy (examining the lowest 25 inches of the bowel) or colonoscopy (examining the entire large bowel).  It is, therefore, not known whether aspirin adds anything to regular (every five years for sigmoidoscopy, every ten years for colonoscopy) screening to remove benign growths (polyps) that could become cancerous, or early cancers.  And neither study evaluated aspirin-induced bleeding into the intestinal tract or brain.  At present, aspirin should not be used for cancer prevention.  We just do not know enough about the benefit to harm ratio or whether aspirin adds anything to regular bowel cancer screening.