Association between tamoxifen treatment and diabetes: a population-based study

Background — There is increasing evidence linking breast cancer and diabetes; however, few studies have explored the association between cancer treatments and risk of diabetes. Tamoxifen may increase diabetes incidence through its estrogen-inhibiting effects. This study assessed whether tamoxifen treatment in older breast cancer survivors is associated with an increased risk of diabetes.

Methods — This nested case-control study used population-based health databases in Ontario, Canada to identify women older than 65 years with early stage breast cancer between April 1, 1996 and March 31, 2006. Cases were defined as cohort members diagnosed with diabetes during follow-up (March 31, 2008), and each case was age-matched with up to 5 controls who did not develop diabetes. After adjusting for other risk factors, the authors compared the likelihood of diabetes between current tamoxifen users and tamoxifen nonusers, based on prescriptions at diabetes diagnosis. They also compared diabetes risk in current aromatase inhibitor users versus nonusers.

Results — Of 14,360 breast cancer survivors identified, mean age 74.9 years, 1445 (10%) developed diabetes over a mean follow-up of 5.2 years. Current tamoxifen therapy was associated with a significantly higher risk of diabetes compared with no tamoxifen therapy (adjusted odds ratio, 1.24; 95% confidence interval, 1.08-1.42; P = .002). There was no association between aromatase inhibitor therapy and diabetes.

Conclusions — Current tamoxifen therapy is associated with an increased incidence of diabetes in older breast cancer survivors. These findings suggest that tamoxifen treatment may exacerbate an underlying risk of diabetes in susceptible women; further studies are needed to better explore this association.