Are pre-menopausal women on tamoxifen at risk for endometrial cancer?

Analysis of endometrial cancer rates in the NSABP P1 trial in women under age 50, demonstrated a relative risk of 1.42 (0.55-3.81) for tamoxifen that was not statistically significant. Per 1000 women, the rates were 0.82 for placebo and 1.16 for tamoxifen. (Fisher et al., PMID 16288118)

The ACOG position is that Tamoxifen does not increase the risk of uterine cancer in premenopausal women:

https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2014/06/tamoxifen-and-uterine-cancer

Five-year therapy with tamoxifen is associated with a 1-2% risk of endometrial cancer.