"Progestogen" is a category, not a molecule. The French E3N cohort followed 80,377 postmenopausal women for a mean of 8.1 postmenopausal years, during which 2,354 invasive breast cancers occurred, and found that the association with breast cancer risk varied significantly by which progestogen was combined with estrogen: relative risk 1.00 for estrogen–progesterone, 1.16 for estrogen–dydrogesterone, and 1.69 for estrogen combined with other progestagens.[1]
Same hormone class, same indication, materially different risk estimates.
The distinction the words hide
Progesterone is the hormone the body makes. Micronized progesterone is that molecule, processed so it can be absorbed when taken orally.
Progestins are synthetic compounds designed to act on the progesterone receptor. They are not the same molecule and they do not have identical effects — several have additional androgenic, non-androgenic or antiandrogenic activity.
Both are progestogens. Both fulfil the endometrial-protection role. In everyday speech, and in a good deal of writing about hormone therapy, they get used interchangeably. The E3N data is the clearest argument that they should not be.
What the study found
Compared with never having used hormone therapy:[1]
| Regimen | Relative risk (95% CI) |
|---|---|
| Estrogen alone | 1.29 (1.02–1.65) |
| Estrogen + progesterone | 1.00 (0.83–1.22) |
| Estrogen + dydrogesterone | 1.16 (0.94–1.43) |
| Estrogen + other progestagens | 1.69 (1.50–1.91) |
Read the intervals rather than the point estimates alone. Estrogen–progesterone at 1.00 with an interval of 0.83–1.22 is a null result — consistent with no increase in risk. Estrogen with other progestagens at 1.69, interval 1.50–1.91, is well clear of 1.0 and is the strongest association in the table.
The authors also examined whether the various "other progestagens" differed among themselves, given their different physiological activities. They did not differ significantly from one another — the split that mattered was progesterone and dydrogesterone on one side, everything else on the other.[1]
What it says about route
The same analysis found no evidence of an association with risk according to the route of estrogen administration — oral or transdermal/percutaneous.[1]
That is worth holding alongside the route findings elsewhere. Route of estrogen matters a great deal for venous clot risk, where first-pass liver metabolism changes clotting proteins. On breast cancer risk in this cohort, it did not. Different outcome, different mechanism, different answer — and a reminder that "transdermal is safer" is not a single claim that transfers across every endpoint.
The authors' conclusion
That the choice of the progestogen component in combined hormone therapy is of importance regarding breast cancer risk, and that it could be preferable to use progesterone or dydrogesterone.[1]
That is appropriately measured language for an observational cohort, and it is the sentence to carry forward.
What this does not establish
- It is observational. E3N is a large prospective cohort, not a randomised trial. Women were not assigned their regimen, and prescribing patterns differ by country, era and clinician in ways that can track other risk factors.
- A mean of 8.1 postmenopausal years. Substantial follow-up, but breast cancer risk from hormone exposure accrues over longer periods than that.
- French prescribing practice. Micronized progesterone has long been more commonly used in France than in the United States, which is part of why this cohort could answer the question at all — and also means the comparison groups reflect that setting.
- It says nothing about endometrial protection. A progestogen's breast-safety profile and its ability to protect the endometrium are separate properties, and they do not necessarily travel together. Which routes and doses of micronized progesterone actually protect the endometrium — including the finding that transdermal does not — is set out in why estrogen comes with progesterone.
How to use this
If a progestogen is needed and there is a genuine choice, this is a reason to ask specifically about micronized progesterone rather than accepting "a progestogen" as a settled decision. It is not a reason to stop an existing regimen that is working, and it is not evidence that synthetic progestins are unsafe — 1.69 is an elevated relative risk against a modest absolute baseline, not a prohibition.
The wider question of what hormone therapy is for, and where the benefit–risk balance turns, is in what menopause hormone therapy actually treats.
Bottom line
In 80,377 women followed a mean of 8.1 postmenopausal years, estrogen combined with micronized progesterone showed no significant increase in breast cancer risk, while estrogen combined with other progestagens showed a relative risk of 1.69. Route of estrogen made no difference to this endpoint. The evidence is observational, but it is large, and its conclusion — that progestogen choice matters and progesterone or dydrogesterone may be preferable — is worth raising with a prescriber.
References
1 source
- Fournier A, Berrino F, Clavel-Chapelon F. Unequal risks for breast cancer associated with different hormone replacement therapies: results from the E3N cohort study. Breast Cancer Res Treat. 2008;107(1):103–11. doi:10.1007/s10549-007-9523-x · PMID 17333341
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