Hormonal Contraceptive Formulations and Breast Cancer Risk in Adolescents and Premenopausal Women.
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IMPORTANCE: Hormonal contraceptives are widely used but how breast cancer risk differs by hormonal content remains unclear. OBJECTIVE: To estimate the difference in breast cancer risk associated with different hormonal contraceptive formulations. DESIGN, SETTING, AND PARTICIPANTS: This Swedish nationwide, population-based cohort study was conducted using linked national registers. All adolescent girls and women aged 13 to 49 years residing in Sweden as of January 1, 2006, with no history of breast cancer, ovarian cancer, cervical cancer, uterine cancer, bilateral oophorectomy, or infertility treatment were included and followed up from 2006 to 2019. Individuals were censored on meeting an exclusion criterion, reaching age 50 years, or study end, whichever occurred first. Data were analyzed from November 2023 to August 2025. EXPOSURE: Ever use and duration of use of hormonal contraceptives, categorized by hormone formulations and route of administration. MAIN OUTCOMES AND MEASURES: Time-dependent Cox regression was used to estimate hazard ratios (HRs) with 95% CIs for incident cases of in situ and invasive breast cancer. RESULTS: Among 2 095 130 adolescent girls and women (median [IQR] age at diagnosis, 45 [41-48] years) who were followed up for 21 020 846 person-years, 16 385 breast cancer cases occurred. Ever use of any hormonal contraceptive was associated with increased breast cancer risk (HR, 1.24; 95% CI, 1.20-1.28), corresponding to 1 additional case per 7752 (95% CI, 5350-14 070) users, with both combined (HR, 1.12; 95% CI, 1.07-1.17) and progestin-only formulations (HR, 1.21; 95% CI, 1.17-1.25) being associated. Higher risk was associated with oral desogestrel-only formulations (HR, 1.18; 95% CI, 1.13-1.23) and oral desogestrel-combined formulations (HR, 1.19; 95% CI, 1.08-1.31), as well as implants containing etonogestrel, desogestrel's active metabolite (HR, 1.22; 95% CI, 1.11-1.35), compared to levonorgestrel-containing combined pills (HR, 1.09; 95% CI, 1.03-1.15) and levonorgestrel, 52 mg, intrauterine system (HR, 1.13; 95% CI, 1.09-1.18). No statistically significant increased risk was observed for medroxyprogesterone acetate injection, etonogestrel vaginal ring, or combined oral drospirenone, despite having many users. CONCLUSIONS AND RELEVANCE: Findings of this cohort study highlight that breast cancer risk varies substantially by progestin content in hormonal contraceptives, providing valuable insights to support more informed contraceptive prescription.