Tamoxifen therapy was linked with reduced risk of contralateral breast cancer (CBC) during and after treatment, according to a retrospective cohort study involving more than 7,500 women. Risk gradually decreased as treatment duration increased.

Participants—all from the Kaiser Permanente systems in Colorado or Oregon—were diagnosed with unilateral breast cancer over a 19-year period ending in 2008. Investigators analyzed patients from initial diagnosis until diagnosis of CBC, other second cancer diagnosis, death, last tumor registry follow-up, exit from the Kaiser plan, or end of study. Among the results:

• 248 women developed CBC after a median 6 months of follow-up.
• The risk decreased substantially with longer therapy duration.
• Current users who took tamoxifen for at least 4 years had an estimated 66% relative risk reduction compared with nonusers.
• Risk reductions were slightly smaller for past users but were still significant at least 5 years after stopping therapy.
• Risk reductions were most clearly seen in women whose primary and contralateral disease were estrogen receptor positive.