At age 41, in 2003, I had LCIS, lobular carcinoma in situ, and ADH atypical ductal hyperplasia, stage 0, pre-cancer. Found on mammogram clustered calcifications. I had a lumpectomy and took tamoxifen for 5 years. Within, 2 years I began to have very very heavy clotted periods. I developed several benign looking ovarian cysts. Finally I had a hysterectomy and was able to stay on the tamoxifen. (After 5 years, my oncologist, switched it to Raloxifene for 7 years.)
The hysterectomy put me into early menopause and either that or the medications caused my osteoporosis at age 58. But it was worth being cancer free for 20 years. The cancer just came back this year. Still very early stage.

Other than that, my only side effect was 10 or 15 lbs that came right off when I finished taking the meds.