I suffered with endometriosis for 30 plus years and had numerous surgeries for it. When they diagnosed me with both breast cancers I was told to take tamoxifen after rads treatment but to check with my gyno first if there would be any issues. Tamox from what I understood makes the lining of your uterus thicken and can cause issues down below if you have pain from endometriosis. I opted not to take tamoxifen and both my oncologist and gynocologist recommended à hysterectomy with ovary removal instead. I had the surgery two months ago, now menopausal and am waiting to see my oncologist to see what, if any meds he recommends now that I am post menopausal. I was too afraid of tamoxifen side effects on uterus in addition to endometriosis, but I was stage 1 IDC and it might be different for you but my oncologist, in my case said it wasn’t that big of a risk on my survival after lumpectomy and rads if I didn’t take it. Glad I had hysterectomy and oophorectomy.

Since it’s a hormone suppression, Tamoxifen will be quite similar to the Lupron they usually give for endometriosis. It too should stop its growth

I too am going to get a hysterectomy and ovaries removed, however never had endometriosis. I took tamoxifen for 20 months and it caused my lining to thicken and created hyperplasia. My oncologist said once the surgery is done we will revisit which meds to be on.. I was stage 1 idc and had double mastectomy.. good luck but I would try a different drug to start. Good luck