Colorectal cancer following tamoxifen therapy for breast cancer (United States)

Background: An analysis combining data from several clinical trials has raised concern that tamoxifen therapy may increase the risk of a subsequent colorectal malignancy. We conducted a nested case–control study to test this hypothesis. Methods: A cohort of women diagnosed with breast cancer in 1978–1992 was identified from the western Washington population-based cancer registry. Cases included women who subsequently developed colorectal cancer prior to 1995. Controls were a random sample of the cohort who did not develop a second primary malignancy, matched to cases on age, stage, and year of initial cancer diagnosis. Tamoxifen use was ascertained by medical record abstraction and physician questionnaires. Results: Thirty-six percent of the 122 cases for whom information could be obtained received tamoxifen versus 38% of 194 controls. Relative to non-users and adjusted for receipt of other therapies, there was no increased risk of colorectal cancer associated with tamoxifen use in general (matched odds ratio mOR] = 0.9, 95% CI 0.6–1.6), longer durations of use (for 37months, mOR = 0.8, 95% CI 0.4–1.9), higher cumulative doses of therapy (for 15 g, mOR = 0.8, 95% CI 0.4–1.6), or use that started at least 5 years prior to the end of study follow-up (mOR = 1.1, 95% CI 0.3–3.7). Conclusions: We found no overall increased risk for colorectal cancer among tamoxifen users.