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Increased risk of breast cancer following different regimens of hormone replacement therapy frequently used in Europe

Epidemiologic studies have shown an increased risk of breast cancer following hormone replacement therapy (HRT). The aim of this study was to investigate whether different treatment regimens or the androgenecity of progestins influence the risk of breast cancer differently. The Danish Nurse Cohort was established in 1993, where all female nurses aged 45 years and above received a mailed questionnaire (n = 23,178). A total of 19,898 women returned the questionnaire (86%). The questionnaire included information on HRT types and regimens, reproductive history and lifestyle‐related factors. Breast cancer cases were ascertained using nationwide registries. The follow‐up ended on 31 December 1999. Women with former cancer diagnoses, women with missing information on HRT, surgical menopause, premenopausal, as well as hysterectomized women were excluded, leaving 10,874 for analyses. Statistical analyses were performed using Cox proportional hazards model. A total of 244 women developed breast cancer during follow‐up. After adjustment for confounding factors, an increased risk of breast cancer was found for the current use of estrogen only (RR = 1.96; 95% CI = 1.16–3.35), for the combined use of estrogen and progestin (RR = 2.70; 95% CI = 1.96–3.73) and for current users of tibolone (RR = 4.27; 95% CI = 1.74–10.51) compared to the never use of HRT. In current users of combined HRT with testosterone‐like progestins, the continuous combined regimens were associated with a statistically significant higher risk of breast cancer than the cyclical combined regimens (RR = 4.16, 95% CI = 2.56–6.75, and RR = 1.94, 95% CI = 1.26–3.00, respectively). An increased risk of breast cancer was noted with longer durations of use for the continuous combined regimens (p for trend = 0.048). The European traditional HRT regimens were associated with an increased risk of breast cancer. The highest risk was found for the use of continuous combined estrogen and progestin. © 2004 Wiley‐Liss, Inc.