Have you heard some version of this in your office?

“I’d like to feel better, but I’m scared of progesterone. Doesn’t it cause breast cancer?”

It’s a fair question, and it's one that used to make me stumble early on.

A lot of women were taught to fear it for many MANY years.

But a 2017 paper by Zhou and colleagues found something many providers like you haven’t seen.

When progesterone met one of the most aggressive breast cancers we know of, it slowed it down...

See, triple-negative breast cancer is the type of cancer everyone worries about.

It grows super fast, comes back often, and likes to spread to the brain. No bueno.

It’s called “triple negative” because it lacks the three receptors we usually test for.

One of them being the classic progesterone receptor.

So for years, the idea was that progesterone couldn’t reach these cells at all.

Let's make an analogy to make this easier to understand.

Picture the cell as a house, and progesterone as a radio broadcast.

The classic progesterone receptor is a radio inside the house.

But with triple-negative cancer has thrown that radio out.

But this research team found that these cells still have an antenna on the roof!

It’s called membrane progesterone receptor alpha, or mPRα. It sits on the outside of the cell and picks up progesterone directly.

So the real question becomes: when progesterone comes through that antenna, what does it tell the cell?

Most people thought it would tell the cancer cells to speed up...

But what actually happened was that the cancer cells slowed down.

See, every one of the 55 triple-negative tumors they examined had the roof antenna (mPRα).

In cancer cells that had it, progesterone slowed growth and pushed more cells into programmed cell death.

But it gets better...

Cancer migration dropped by about 43%, and cancer invasion dropped by about 39%!

In mice, cancer cells with the antenna didn’t form brain metastases once progesterone was given.

Then came the most convincing part.

When the researchers removed the antenna, progesterone stopped working.

When they added the antenna to cells that barely had it, progesterone started working.

That’s about as clean a cause-and-effect signal as basic science gets. The antenna picked up the progesterone, and the progesterone hit the brakes.

Now, to be clear, this was done in cells and mice, not people. It doesn’t make progesterone a cancer treatment (yet), and it isn’t a reason to prescribe it for that.

It’s also specific to bioidentical progesterone.

Synthetic progestins are different molecules, and none of this should be applied to them.

But it does shift the conversation in the right direction.

Most of the fear around progesterone and breast tissue traces back to synthetic progestins, not the hormone your body makes.

When researchers study real progesterone, the signal often looks different. In this case, it looked protective.

And this isn’t only a cell-and-mouse story.

The French E3N cohort followed more than 80,000 postmenopausal women for about 8 years. France is one of the few places where large numbers of women used micronized progesterone instead of synthetic progestins, so researchers could compare them directly.

Here’s what they found.

Women using estrogen with a synthetic progestin had about 69% higher breast cancer risk than women who never used hormones.

Women using estrogen with micronized progesterone had no measurable increase in breast cancer.

Their relative risk was 1.00.

Same estrogen. Different progesterone molecule. Very different signal.

Now, that's observational data, so confounding is always possible.

But put it next to the Zhou paper and a pattern starts to show up.

In the lab, progesterone slowed aggressive cancer cells. In real women, it didn’t carry the risk the synthetics did.

So the next time a patient asks whether progesterone causes breast cancer, you don’t have to choose between false reassurance and fear.

You can tell her the truth: the hormone itself appears to behave more like a brake than an accelerator, both in some of the most aggressive breast cancer cells we study and in the largest real-world comparison we have.

That’s a very different conversation than the one most of us were trained to have.

Hope this helps.

-Nico

PS

Helping providers understand how to separate progesterone from the drugs that gave it a bad name is a big part of what we teach inside the HRTU Master Course -> hrtuniversity.com/hrtcourse​

​References:​

​Zhou L, et al. Progesterone suppresses triple-negative breast cancer growth and metastasis to the brain via membrane progesterone receptor α. Int J Mol Med. 2017;40(3):755-761. doi:10.3892/ijmm.2017.3060​

​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):103-111. doi:10.1007/s10549-007-9523-x​