Do pre-menopausal women usually gain weight on tamoxifen?

No: There have been many studies that have compared tamoxifen to placebo, demonstrating it's effectiveness treating breast cancer; none of these studies, to my knowledge, ever showed any difference between tamoxifen and placebo regarding weight gain. Nevertheless, as many as 10% of women on tamoxifen do report weight gain. This may be due to post-chemotherapy effects or menopause, as well.

Yes: An unfortunate side effect of tamoxifen is weight gain. Exercise and a healthy diet are very important in patients w breast cancer, especially if taking tamoxifen.