Breast Cancer and Levonorgestrel-Releasing Intrauterine Systems

To the Editor We believe that a recent Research Letter presented an overly strong interpretation of highly nuanced data that had residual confounding. The relationship between hormone therapies and breast cancer is complex, beginning with choosing comparator groups. Nonuse of hormonal contraception is not a base health state, but rather a culmination of health care access, preference, and need, which render this comparator biased in its own right. This does not preclude analyses comparing these groups, but necessitates intentionality in study design and thoughtful interpretation. The ideal comparator group for hormonal intrauterine device (IUD) users is unknown but needs to be situated within social and biologic plausibility. Of note, a study of 5113 patients with breast cancer diagnosed from 2000 to 2007 and 20 452 matched controls reported no increased risk in breast cancer in those using levonorgestrel-releasing IUDs compared with copper IUDs.