Menopause Education Series
A Deeper Dive into Progesterone
Progesterone is more than a uterine protectant. It's a neurosteroid precursor with unique sleep and mood effects. But not all progestogens are the same. This guide explains the biology, the critical distinction between progesterone and progestins, and what the evidence actually shows.
The Biology of Progesterone
Progesterone does a lot more than protect your uterus. It helps you sleep, supports your bones, and balances out estrogen. Understanding how it works explains why losing it matters, and why the type of progesterone you take matters.
Progesterone vs Progestins
They share a receptor. They share almost nothing else.
For too long, doctors and the media have treated all types of progesterone-like drugs as if they were the same thing. They are not. Micronized progesterone (the natural, bioidentical form) and synthetic progestins like Provera (MPA) are very different medications with very different effects on your body.
| Feature | Micronized Progesterone (Prometrium) | MPA (Provera) | Norethindrone/NETA | Dydrogesterone | LNG-IUD (Mirena) |
|---|---|---|---|---|---|
| Bioidentical | Yes | No | No | No (retro-isomer) | No |
| Androgenic activity | None | Weak | Moderate | None | Moderate (local) |
| Glucocorticoid activity | None | Moderate | None | None | None |
| Allopregnanolone production | Yes (oral) | No | No | No | No |
| Sleep benefit | Yes (oral) | No | No | No | No |
| HDL effect | Neutral (PEPI) | Reduces HDL | Reduces HDL | Neutral | Minimal |
| Breast cancer (observational) | No increase E3N 5–7 yrs | Increased (WHI, E3N) | Increased (E3N) | Not significant | Insufficient data |
| Endometrial protection | Yes | Yes | Yes | Yes | Yes (excellent local) |
| VTE risk | Neutral (ESTHER) | May increase | Insufficient data | Insufficient data | Minimal systemic |
| Available in US | Yes (generic) | Yes (generic) | Yes | No | Yes (off-label for HRT) |
WHI tested MPA, not progesterone
The breast cancer, cardiovascular, and metabolic findings from the WHI combined arm cannot be attributed to micronized progesterone. Applying those results to progesterone is a pharmacologic error.
The PEPI trial established the distinction
The PEPI trial (1995) was the first major RCT to show clinically meaningful differences: micronized progesterone preserved estrogen’s favorable HDL effect significantly better than MPA.
Formulations & Routes
I think about progesterone formulations in terms of what they do systemically versus locally, and whether you want the sleep benefit or not. That distinction drives the choice.
Oral Micronized Progesterone (Prometrium)
This is natural progesterone ground into tiny particles so your body can absorb it. When you swallow it, your liver processes it and converts some of it into allopregnanolone, the calming brain chemical that helps you sleep. That’s why I have patients take it at bedtime. Contains peanut oil.
Dosing: 200 mg/day for 12-14 days/month (cyclic) or 100 mg/day daily (continuous)
Vaginal Progesterone
Crinone gel (4%, 8%), oral capsules used vaginally (off-label)
When progesterone is given vaginally, it goes directly to the uterus without passing through the liver first. This means higher progesterone levels where you need them (the uterine lining) and much less of the sleepy effect, since less allopregnanolone is produced.
LNG-IUD (Mirena)
A small device placed in the uterus that releases a progestin (levonorgestrel) directly where it’s needed. Very little of the hormone enters your bloodstream. More and more doctors are using it to protect the uterine lining while women take estrogen therapy.
Synthetic Oral Progestins (MPA, Norethindrone)
These are man-made versions of progesterone. They protect the uterine lining, but they come with more side effects than micronized progesterone. MPA (Provera) is what was used in the WHI study that scared many women away from hormones.
My default
I prescribe oral micronized progesterone at bedtime as my default. The sleep benefit is real, the safety profile is favorable, and it’s available as an affordable generic. If sedation is intolerable, vaginal progesterone or the LNG-IUD are good alternatives.
Safety & Side Effects
The safety profile depends entirely on which progestogen you’re taking.
The safest progestogen regimen based on current evidence: oral micronized progesterone combined with estrogen patches or gel. This combination avoids the liver-related risks of estrogen pills and the unwanted side effects of synthetic progestins.