The 52 mg levonorgestrel-releasing intrauterine system (LNG-IUS) is a highly effective method of contraception, but its use is often associated with early “nuisance bleeding” which may lead to discontinuation. Many medications have been studied with progestin-only contraceptives such as the LNG-IUS to manage irregular bleeding. The only study to demonstrate a sustained reduction in bleeding lasting for two months was the selective estrogen receptor modulator (SERM) tamoxifen in users of the levonorgestrel contraceptive implant. The proposed research project will be the first to test the effectiveness of tamoxifen to improve bleeding patterns in users of the LNG-IUS. This is a randomized, controlled, double blind clinical trial enrolling new users of the LNG-IUS to try to improve frequent or prolonged bleeding after insertion. Participants are randomized to receive tamoxifen 10mg or placebo twice daily for seven days starting 21 days after IUS insertion. All participants will maintain daily bleeding diaries via text or email message and on paper for a total of 51 days, including three weeks of baseline data and 30 days following initiation of treatment with study drug. The primary anticipated outcome is that women who receive tamoxifen will have a reduction in bleeding/spotting days compared to placebo in the 30 days after starting study drug. This may lead to improved satisfaction with and continuation of the LNG-IUS. Given that the LNG-IUS is 20 times more effective at pregnancy prevention than oral contraceptive pills, strategies to increase its acceptability will translate into improved prevention of unplanned pregnancies.