Does taking tamoxifen benefit women with cdis?

Yes. Women with a history of ductal-carcinoma-in-situ (dcis) may have a 50% reduction in developing a second breast cancer when taking tamoxifen for 5 years ("chemoprevention"). Furthermore, following a lumpectomy, the combination of tamoxifen and radiation therapy may have a decreased risk of cancer recurrence in the affected breast greater than either therapy alone.
Yes. When you have hormone sensitive (er/pr +) dcis, tamoxifen is the appropriate endocrine therapy. In fact, it's the only hormonal therapy for this type of cancer at this time. Tamoxifen will also reduce the risk of developing a different breast cancer by 50%.