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Comparison of side-effect profiles during active treatment versus follow-up in the International Breast Cancer Intervention Study I tamoxifen prevention trial

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Tamoxifen is an effective drug but its role in prevention is limited by its side-effect profile, particularly related to endometrial problems and thrombotic events. We present updated results on the comparison of side effects between active treatment and the follow-up period incorporating four additional years of follow-up.


In the International Breast Cancer Intervention Study I (IBIS-I) study, 7,154 women at increased risk of breast cancer were either randomised to tamoxifen 20 mg/day or placebo for 5 years. Women gave detailed information of any side effects at each 6-monthly follow-up visit and once a year after active treatment.


During active treatment, large numbers of side effects were reported by participants in both treatment arms. However, the only major categories that showed differences were vasomotor and gynaecological side effects, which were about 12% higher in the tamoxifen group than the placebo group. Thromboembolic events were significantly higher in the tamoxifen group compared with the placebo group (85 versus 42, P < 0.001), whereas myocardial infarctions were reduced in women on tamoxifen during active treatment (2 versus 7, P = 0.09). Women on tamoxifen developed significantly more endometrial cancer during active treatment than women on placebo (12 versus 3, P = 0.02). After active treatment, most menopausal-like side effects decreased and the differences between the two treatment arms were not significant. Reports on vaginal discharge decreased after stopping tamoxifen. Thromboembolic events were similar between the two treatment arms after active treatment (32 versus 26, P = 0.4). No significant difference in endometrial cancer between the two treatment groups was found during follow-up time (8 versus 5, P = 0.4).


Although large numbers of side effects were reported, tamoxifen was well tolerated and no new safety concerns were identified. For most side effects no significant difference between treatment groups was found after ceasing tamoxifen.

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  • Breast Cancer
  • Placebo
  • Myocardial Infarction
  • Tamoxifen
  • Placebo Group