Progesterone

It prepares and stabilizes the endometrium, supports early pregnancy, and acts in the brain partly through its neurosteroid metabolite allopregnanolone, a positive allosteric modulator of the GABA-A receptor — which is why oral progesterone is sedating.

The pharmaceutical form described here is bioidentical micronized progesterone (brand Prometrium / Utrogestan): the same molecule as the body's own, micronized and oil-suspended to improve absorption, and taken orally (also used vaginally in assisted reproduction). It is a prescription drug, not a dietary supplement.

Its evidence is genuinely strong but indication-bounded.

First and most important, progesterone is the essential endometrial-protection component of menopausal hormone therapy: estrogen given alone to a woman with a uterus drives endometrial hyperplasia and raises endometrial-cancer risk, and adding a progestogen prevents that.

The PEPI trial randomized 596 postmenopausal women to placebo, estrogen alone or estrogen plus a progestogen (including cyclic micronized progesterone 200 mg/day) for 3 years, and the progestogen arms had hyperplasia rates similar to placebo where estrogen alone produced hyperplasia.

PEPI measured hyperplasia, not cancer, and one review (Prior 2015) reports that endometrial cancer is increased with estrogen plus cyclic oral micronized progesterone at 200 mg/day, so cancer protection is not established for every regimen.

Second, progesterone is standard luteal-phase support in IVF/assisted reproduction: a Cochrane review (van der Linden 2015) found higher live birth or ongoing pregnancy rates with progesterone than with placebo or no treatment (OR 1.77, 95% CI 1.09-2.86, 5 RCTs, 642 women), on very low-quality evidence.

Third, oral micronized progesterone is widely used off-label for menopausal sleep and vasomotor symptoms.

A randomized, placebo-controlled study in 8 healthy postmenopausal women (Caufriez 2011) found progesterone 300 mg had no effect on undisturbed sleep but reduced the sleep disturbance caused by overnight blood sampling, and a placebo-controlled RCT in healthy postmenopausal women (Hitchcock & Prior 2012) found oral micronized progesterone reduced vasomotor symptoms (hot flushes/night sweats) versus placebo.

Fourth, in early-pregnancy bleeding the large PRISM trial (Coomarasamy 2019, NEJM) found vaginal progesterone 400 mg twice daily did NOT significantly improve live births (75% vs 72%, P=0.08) in 4,153 women.

Its abstract reports no subgroup result, so it does not support a blanket 'progesterone prevents miscarriage' claim. Progesterone also treats secondary amenorrhea by inducing a withdrawal bleed.

A crucial nuance runs through all of this: micronized (bioidentical) progesterone is not the same as the older synthetic progestins.

The Women's Health Initiative breast-cancer signal came from a combined regimen using a synthetic progestin (medroxyprogesterone acetate, MPA), and observational data (the E3N cohort, Fournier 2008) found that estrogen combined with micronized progesterone was associated with little or no increased breast-cancer risk over several years, in contrast to estrogen plus synthetic progestins — so the WHI signal should be attributed to synthetic progestins, not assumed for micronized progesterone, while acknowledging that long-term randomized breast-outcome data on micronized progesterone are more limited.

The trade-offs are real: oral progesterone commonly causes drowsiness, dizziness, and fatigue (it is taken at bedtime for exactly this reason), and can cause mood changes, bloating, and breast tenderness.

Oral micronized progesterone also has low oral bioavailability because of extensive first-pass hepatic metabolism, which is part of why it is dosed relatively high and at night and why vaginal dosing is preferred for IVF.

Finally, the OTC 'progesterone creams' sold as wellness products absorb erratically and unreliably and are not equivalent to prescription oral micronized progesterone.

The score reflects strong, randomized evidence for its specific indications — endometrial protection, IVF luteal support, sleep, and vasomotor relief — set against the facts that these benefits are indication-specific rather than a general enhancement, that it is prescription-only with sedation/mood trade-offs, and that the long-term breast-safety picture for micronized progesterone, while reassuring relative to synthetic progestins, is not as deeply evidenced.