BACKGROUND: The copper intrauterine device (IUD) can cause side effects in some women; increased uterine bleeding and pain may cause early removal. Because of simplified reporting from previous research, little is known about how side effects might change over time. STUDY DESIGN: This is a secondary analysis of a prospective study of 1947 first-time copper IUD users. Over a 1-year period, we collected detailed information on side effects and looked for trends using generalized mixed-effects regression modeling. RESULTS: During menses, most bleeding and pain side effects were found to decrease over time (p<.05). During intermenstrual intervals, overall spotting and pain complaints remained unchanged, but the number of days with these problems increased (p