Regardless of brand, Tamoxifen can be challenging. I don't know the scientific part of it, but I do wonder how the body metabolises Tamoxifen and why the hidden extras, side effects! As others have said already, counteract with exercise and or dietary changes may help. As long as it is doing what it is meant to do!
Copy and paste from https://www.bcna.org.au/understanding-breast-cancer/treatment/hormone-therapy/
"...Hormone therapy works by stopping oestrogen from ‘feeding’ the breast cancer cells. It has been found to be very effective in reducing the risk of the cancer spreading to other parts of the body, or of a new breast cancer developing in the same or other breast..."
I live in hope that it is doing what it is meant to do as my body creaks, moans and I groan occasionally! Due for review again in 10 days so one assumes it is doing it's work!
Take care