Alternate-day, low-dose aspirin and cancer risk: long-term observational follow-up of a randomized trial.


This study looked at whether taking aspirin every other day may reduce the risk of cancer, particularly cancer of the colon and rectum. The study was initially designed as a clinical trial to look at risk of cancer, heart disease and stroke in healthy women. Originally, the participants were randomly assigned to receive aspirin or not, but then were followed for an additional 10 years after the completion of the clinical trial to monitor the occurrence of cancer and bleeding.


The 39,876 participants were women 45 years or older, in healthcare professions. In 1993-6, they were randomly assigned to two groups: 1) 100 mg of aspirin every other day or 2) placebo. These women were followed for 10 years during the initial study. After the clinical trial ended, 84% of the women continued to be followed for 8 more years, but generally did not continue their initial assignment to take aspirin or not. The authors looked at whether initial aspirin use changed the risk of different types of cancer as well as the risk of bleeding complications.


  • Women who took aspirin every other day had a 20% reduction in the risk of colorectal cancer. Various characteristics of the women (such as age, smoking and obesity) did not alter the favorable impact of aspirin on colorectal cancer.

  • Reduced colorectal cancer with aspirin was only apparent after 10-years. There was no difference in colorectal cancer during the initial clinical trial, but only after the additional 8 years of extended follow-up.

  • Lung and breast cancer were not reduced by taking aspirin. Total cancer risk was 3% less among those who took aspirin, but this result may be due to random chance. There were no differences in rates of dying from cancer or other causes between those who took or did not take aspirin.

  • Women who took aspirin were 17% more likely to have gastrointestinal bleeding and 14% more likely to have stomach ulcers compared to those who did not. This negative effect of aspirin was only found during the clinical trial and not during the extended 8-year follow-up.


  • Every other day, low-dose, long term aspirin use reduced the chance of developing colon and rectal cancer in women. This benefit occurred across a broad range of participants. While bleeding risks need to be taken into account, this study suggests that colorectal cancer prevention should be considered along with aspirin’s cardiovascular disease benefits.

  • Long-term use of aspirin showed greatest benefit. Prevention of colon and rectal cancer using aspirin is not immediate, but is delayed and is apparent only after a decade of aspirin use.