Tamoxifen?

Hi Ladies, I'm still doing research about Tamoxifen and how it affects those of us with HER2. I was wondering if any of you out there have taken Tamoxifen and then discontinued it. Why did you discontinue it? If it stopped working for you HOW did you determine it had stopped working? What were the signs? My Onc has been very evasive about how he knows it is working for me. I would hate to have disease progression be the indication that it wasn't working. Do you know of any tests that can be done? Thanks in advance for any info you can give me. Take Care, Linda