Levonorgestrel-releasing intrauterine system linked to increased breast cancer risk

Exposure to the levonorgestrel-releasing intrauterine system, especially during the first few years, appears to contribute to a heightened risk of breast cancer among women with abnormal uterine bleeding, endometriosis, or leiomyomas, according to a retrospective study.

Researchers used the Korean National Health Insurance Claims database. They identified 2,094,029 women aged 30–49 years with initial diagnoses of endometriosis, uterine leiomyomas, or abnormal uterine bleeding for inclusion in the analysis.

Propensity score matching was used to establish balanced cohorts of levonorgestrel-releasing intrauterine system users and nonusers. Cox proportional hazards models were used to examine the association between levonorgestrel-releasing intrauterine system exposure duration and breast cancer incidence.

The propensity-matched cohort included 61,010 women. Breast cancer occurred more frequently among levonorgestrel-releasing intrauterine system users than among nonusers (223 vs 154 cases per 100,000 person-years). Use of LNG-IUS was associated with a 38-percent increase in breast cancer risk (hazard ratio [HR], 1.38, 95 percent confidence interval [CI], 1.192–1.585).

Analysis of levonorgestrel-releasing intrauterine system exposure over time showed that early initiation was associated with an increased risk of breast cancer, which may decrease over time (less than 3 years of exposure: HR, 5.40, 95 percent CI, 4.037–7.216 with early initiation and HR, 1.04, 95 percent CI, 0.547–1.994 with late initiation; 3–4.9 years: HR, 3.44, 95 percent CI, 2.671–4.431 with early initiation and HR, 1.75, 95 percent CI, 1.12–2.723 with late initiation; 5 years or more: HR, 1.77, 95 percent CI, 1.26–2.479).