Association between frequent use of nonsteroidal anti-inflammatory drugs and breast cancer

Background: Eighty percent of all breast cancers and almost 90% of breast cancer deaths occur among post-menopausal women. We used a nested case control design to examine the association between nonsteroidal anti-inflammatory drug (NSAID) use and breast cancer occurrence among women over 65 years of age. The cyclooxygenase (COX)-2 enzyme is expressed more in breast cancers than in normal breast tissue. COX-2 inhibition may have a role in breast cancer prevention. Methods: In the Canadian province of Quebec, physician services are covered through a governmental insurance plan. Medication costs are covered for those >= 65 years of age and a publicly funded screening program for breast cancer targets all women 50 years of age or older. We obtained encrypted data from these insurance databases on all women >= 65 years of age who filled a prescription for COX-2 inhibitors, non-selective NSAIDs (ns-NSAIDs), aspirin, or acetaminophen between January 1998 and December 2002. Cases were defined as those women who have undergone mammography between April 2001 and June 2002 and had a diagnosis of breast cancer within six months following mammography. Controls included those who have undergone mammography between April 2001 and June 2002 without a diagnosis of any cancer during the six months following mammography. The exposure of interest, frequent NSAID use, was defined as use of ns-NSAIDs and/or COX-2 inhibitors for >= 90 days during the year prior to mammography. Frequent use served as a convenient proxy for long term chronic use. Results: We identified 1,090 cases and 44,990 controls. Cases were older and more likely to have breast cancer risk factors. Logistic regression models adjusting for potential confounders showed that frequent use of ns-NSAIDs and/or COX-2 inhibitors was associated with a lower risk of breast cancer (OR: 0.75, 95% confidence interval 0.64-0.89). Results were similar for COX-2 inhibitors (0.81, 0.68-0.97) and ns-NSAIDs (0.65, 0.43-0.99), when assessed separately. Frequent use of aspirin at doses >100 mg/day in the year prior to mammography was also associated with a lower risk of breast cancer (0.75, 0.64-0.89). However, use of aspirin at doses <= 100 mg/day did not have any association with breast cancer (0.91, 0.71-1.16). Conclusion: Women who use NSAIDs or doses of aspirin >100 mg frequently may have a lower risk of breast cancer.