Breast Cancer, Prevention - Hi, I have been diagnosed with DCIS, oestrogen receptor positive. I am?

This is quite a specialized question and your oncologist is probably the person you should be getting advice from.

However, theoretically, if you have estrogen receptor positive DCIS it is probably good to take Tamoxifen.

Tamoxifen blocks the effects of estrogen, which can help breast cancer cells survive and grow. Most women with estrogen sensitive breast cancer benefit from this drug. A newer class of medicines called aromatase inhibitors, such as exemestane (Aromasin), have been shown to work just as well or even better than tamoxifen in post-menopausal women with breast cancer.

Targeted therapy, also called biologic therapy, is a newer type of cancer treatment. This therapy uses special anti-cancer drugs that identify certain changes in a cell that can lead to cancer. One such drug is trastuzumab (Herceptin). For women with stage IV HER2-positive breast cancer, Herceptin plus chemotherapy has been shown to be work better than chemotherapy alone. Studies have also shown that in women with early stage HER2-positive breast cancer, this medicine plus chemotherapy cuts the risk of the cancer coming back by 50%.

http://www.drugs.com/enc/breast-cancer.html