In premenopausal survivors, tamoxifen was linked to fivefold endometrial hyperplasia
A Taiwanese cohort of 26,000 women aged 20 to 50 puts numbers on a known tamoxifen risk in a population — younger, premenopausal — where data has been thinner.
Tamoxifen's association with endometrial cancer in postmenopausal women is well established. A study published this week in Obstetrics & Gynecology quantifies the same risk in a population where it's been studied less: premenopausal women, using a Taiwanese national insurance database covering more than 26,000 women [s1].
The design
Researchers used Taiwan's National Health Insurance claims data, linked to the national cancer registry, to build a retrospective cohort using a target trial emulation framework — a method meant to approximate a randomized comparison using observational data [s1]. They identified women aged 20 to 50 diagnosed with estrogen receptor-positive breast cancer who had a mastectomy or lumpectomy between 2010 and 2019, excluding those with a prior hysterectomy, neoadjuvant therapy, or existing uterine disease [s1]. Tamoxifen users were defined as those who received it as adjuvant hormone treatment within a year of surgery; inverse probability weighting was used to balance the two groups on baseline characteristics [s1]. The final cohort comprised 23,062 tamoxifen users and 3,000 nonusers [s1].
What it found
Over the follow-up period, uterine disease developed in 3,889 tamoxifen users and 109 nonusers, including 106 cases of endometrial cancer among users and 5 among nonusers [s1]. In the intention-to-treat analysis, tamoxifen use was associated with a hazard ratio of 4.15 (95% CI 2.65–6.50) for endometrial polyps, 5.42 (95% CI 4.09–7.18) for endometrial hyperplasia, and 2.41 (95% CI 0.86–6.72) for endometrial cancer [s1]. A per-protocol analysis, which more closely tracks women who actually continued tamoxifen as prescribed, produced larger estimates: 4.75 (95% CI 2.55–8.86) for polyps, 8.37 (95% CI 5.24–13.35) for hyperplasia, and 4.20 (95% CI 1.20–14.63) for endometrial cancer [s1]. Risk of uterine disease overall rose with longer duration of tamoxifen use [s1].
Reading the confidence intervals
The endometrial cancer estimate is the one to read carefully. In the intention-to-treat analysis, its 95% confidence interval spans from 0.86 to 6.72 — meaning the result does not reach conventional statistical significance in that analysis, since the interval crosses 1.0. The per-protocol estimate (4.20, 95% CI 1.20–14.63) does clear that threshold, but its interval is wide, running from a modest 20% increase up to more than fourteenfold, reflecting the relatively small number of cancer cases — 106 among users, 5 among nonusers — that the estimate is built on. The hyperplasia and polyp findings, based on larger case counts, are more statistically stable.
What this doesn't establish
This is an observational cohort from a single national healthcare system, and while the researchers used methods designed to approximate a trial, unmeasured differences between women who received tamoxifen and those who didn't — the nonuser group is far smaller, at 3,000 versus 23,062 users, which may itself reflect clinical selection — can still bias the comparison. The population is specifically premenopausal Taiwanese women with ER-positive breast cancer; whether the magnitude of risk applies to other ethnic populations or treatment eras is not addressed here. Absolute risk also matters alongside the relative risk: endometrial cancer occurred in 106 of 23,062 tamoxifen users, or roughly 0.46% over the study's follow-up window, a rate the paper does not annualize.
Tamoxifen's benefit in reducing breast cancer recurrence in ER-positive disease is not in question and is not what this study measured. The authors frame their finding as supporting closer monitoring of the uterus in tamoxifen users within this age range, not as an argument against the drug's use.
What to watch
Whether the estimates hold up in cohorts from other countries and health systems, and whether the endometrial cancer signal — the least statistically stable of the three uterine outcomes measured here — strengthens or weakens as case numbers accumulate with longer follow-up. This article is not medical advice; treatment decisions about tamoxifen involve weighing its recurrence-reduction benefit against risks it carries, a judgment for a patient and her clinician.
Sources
- Tamoxifen Use and Risk of Uterine Diseases in Young Women With Breast Cancer — Obstetrics & Gynecology, 5 March 2026
Sources
- Tamoxifen Use and Risk of Uterine Diseases in Young Women With Breast Cancer — Obstetrics & Gynecology , March 5, 2026
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