Background: Epidemiologic evidence indicates that aspirin use is associated with reduced risks of colon cancer and possibly several other cancers, including prostate and breast cancers. Recent results from the Women's Health Study randomized trial indicate that long-term use of low-dose aspirin (100 mg every other day) does not substantially reduce cancer risk. However, the potential effect of long-term daily use of higher doses of aspirin on cancer incidence remains uncertain.
Methods: We examined associations between long-term daily use of adult-strength aspirin (> or = 325 mg/day) and both overall cancer incidence and incidence of 10 types of cancer among 69,810 men and 76,303 women participating in the Cancer Prevention Study II Nutrition Cohort, a relatively elderly population. Aspirin use was reported at enrollment in 1992-1993 and updated in 1997, 1999, and 2001. Multivariable Cox proportional hazards regression was used to calculate rate ratios (RRs).
Results: During follow-up through June 2003, 10,931 men and 7196 women were diagnosed with cancer. Long-term (> or = 5 years) daily use of adult-strength aspirin, compared with no use, was associated with lower overall cancer incidence in men (multivariable-adjusted RR = 0.84, 95% confidence interval [CI] = 0.76 to 0.93) and non-statistically significantly lower overall cancer incidence in women (multivariable-adjusted RR = 0.86, 95% CI = 0.73 to 1.03). Overall cancer incidence per 100,000 person-years (standardized to the age distributions of men and women in the study) with long-term daily aspirin use and no aspirin use was 1858 and 2163, respectively, among men and 1083 and 1169, respectively, among women. Long-term daily aspirin use was associated with lower incidence of colorectal cancer (RR = 0.68, 95% CI = 0.52 to 0.90 among men and women combined) and prostate cancer (RR = 0.81, 95% CI = 0.70 to 0.94) and a non-statistically significant lower risk of female breast cancer (RR = 0.83, 95% CI = 0.63 to 1.10).
Conclusions: Long-term daily use of adult-strength aspirin may be associated with modestly reduced overall cancer incidence in populations among whom colorectal, prostate, and breast cancers are common.