I finished radiation for breast cancer dcis 2 weeks ago . im on tamoxifen now no side effects yet but i do have redness under breast how do i know if its reaction to radiation or tamoxifen?

Some redness on the skin of the breast is expected after Radiotherapy. Tamoxifen does not have this side effect. So I would advise you to watch the red area and keep it dry(avoid sweating) and expect resolution redness over the next 2-3 weeks. There is sometimes yeast(Candida)infection under the breast which may require antibiotic treatment. Consult your radiation oncologist if the redness worsens