Estrogen given on its own stimulates the lining of the uterus, and sustained stimulation raises the risk of endometrial hyperplasia and cancer. That is why a woman with an intact uterus taking systemic estrogen is given a progestogen alongside it. It is also the one boxed warning the FDA kept in place when it removed the others from menopausal hormone therapy in November 2025.[1]
The part that gets glossed over is that not every form of progesterone does this job. A systematic review of micronized progesterone reached three separate conclusions for three routes, and one of them is negative.[2]
The requirement
International guidelines agree that postmenopausal women with an intact uterus using estrogen therapy should receive a progestogen for endometrial protection.[2] What those guidelines have generally not specified is progestogen type, dosage, route of application, or duration of safe use — which is the gap the review set out to fill for micronized progesterone specifically.
If you have had a hysterectomy, this does not apply to you. There is no endometrium to protect, which is why estrogen-alone therapy exists as a category and why the FDA's retained boxed warning is written specifically for systemic estrogen-alone products in women with a uterus.[1]
What the review concluded, by route
An international expert panel, working from a systematic literature review, set out three recommendations:[2]
| Route | Verdict | Regimen |
|---|---|---|
| Oral micronized progesterone | Provides endometrial protection | Sequentially, 12–14 days/month at 200 mg/day, for up to 5 years |
| Vaginal micronized progesterone | May provide endometrial protection | Sequentially, at least 10 days/month at 4% (45 mg/day), or 100 mg/day every other day, for up to 3–5 years — off-label use |
| Transdermal micronized progesterone | Does not provide endometrial protection | — |
Three things follow that are worth stating without hedging.
Oral is the route with the affirmative evidence, at a specified dose, for a specified number of days per month, for a specified duration. "Progesterone" is not a single intervention — 200 mg for 12–14 days a month is the thing that was studied.
Vaginal is "may," and off-label. Both qualifiers are the panel's, not ours.
Transdermal does not do it. This is the clearest and most consequential of the three. Progesterone creams, gels and sprays applied to skin are widely sold, including in contexts that imply they balance estrogen. On the evidence in this review, transdermal micronized progesterone should not be relied on to protect the endometrium of a woman taking systemic estrogen.
That finding applies to transdermal progesterone as a route, whoever makes it — including any transdermal progesterone product Keen may offer in future. We would rather publish the constraint than discover a reader assumed otherwise.
Why "sequential" matters
Note that all the protective regimens are sequential — a defined block of days each month, not continuous low-level exposure. The endometrium responds to a pattern, and the pattern is part of the protection. A dose spread thinly across every day is not the same intervention as 200 mg for a fortnight, even if the monthly total looked similar.
Duration
The oral recommendation carries "for up to 5 years," and the vaginal one "for up to 3–5 years." Those are the periods the evidence covers, not the point at which something goes wrong. What lies beyond them is unstudied rather than unsafe, and it is a conversation to have with a prescriber rather than a deadline.
Where this sits
- The requirement itself, and the boxed warning that survived the 2025 labelling change, are covered in the FDA's 2025 hormone therapy labelling change.
- Which progestogen you use also affects breast cancer risk, separately from endometrial protection — see micronized progesterone versus synthetic progestins.
- Low-dose vaginal estrogen is a different situation. Position-statement guidance is that a progestogen is not indicated with it, though endometrial safety beyond a year is unstudied — see vaginal estrogen for genitourinary symptoms.
Bottom line
Systemic estrogen in a woman with a uterus requires a progestogen, and that requirement did not change when the FDA removed the other boxed warnings. For micronized progesterone specifically, oral dosing at 200 mg/day for 12–14 days a month has the affirmative evidence; vaginal dosing may work and is off-label; transdermal does not provide endometrial protection. Route and regimen are not details here — they are the difference between protection and its absence.
References
2 sources
- U.S. Department of Health and Human Services; U.S. Food and Drug Administration. HHS Advances Women's Health, Removes Misleading FDA Warnings on Hormone Replacement Therapy. November 10, 2025. fda.gov
- Stute P, Neulen J, Wildt L. The impact of micronized progesterone on the endometrium: a systematic review. Climacteric. 2016;19(4):316–328. doi:10.1080/13697137.2016.1187123 · PMID 27277331
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