What Is Progesterone? Dosage, Half-Life & How to Track It

What progesterone is and how it works

Progesterone is a steroid hormone produced by the corpus luteum after ovulation (and by the placenta during pregnancy) whose core job is preparing and maintaining the uterine lining for a possible pregnancy. In hormone therapy, its most common role is essential rather than glamorous: opposing estrogen’s proliferative effect on the endometrium. Estrogen alone stimulates the uterine lining to thicken; without progesterone to balance it, that unchecked growth raises the risk of endometrial hyperplasia and, over time, cancer — which is why anyone with a uterus on estrogen therapy is prescribed progesterone alongside it.

Progesterone also has effects outside the uterus. One of its metabolites, allopregnanolone, acts on GABA-A receptors in the brain — the same receptor system targeted by benzodiazepines — producing a mild calming, sedative effect. That single fact drives one of the most consistent pieces of dosing advice in this field: take oral progesterone at bedtime.

It shows up in a few different contexts: as the progestogen component of menopausal HRT, as luteal-phase support in fertility treatment, and — less commonly, with thinner evidence for benefit — in some feminizing gender-affirming regimens, where its role is more debated than estrogen’s or spironolactone’s.

Dosing by route: oral, vaginal & cream

Oral micronized progesterone is dosed in fixed-strength capsules, most often 100 mg or 200 mg. A common menopausal HRT pattern uses 100 mg nightly in a continuous-combined regimen (progesterone every day alongside daily estrogen) or 200 mg nightly for 12–14 days per cycle in a cyclic regimen — the cyclic approach is timed specifically to reliably shed the lining each month rather than let it accumulate.

Vaginal and rectal progesterone (used more in fertility settings for luteal support) skip first-pass liver metabolism entirely, delivering more progesterone directly to the uterus with less of the oral form’s systemic sedation. Progesterone cream, by contrast, is compounded and applied to skin — it’s common in some HRT regimens, but its absorption into the bloodstream is inconsistent and far less rigorously studied than either oral or vaginal progesterone, a real, documented limitation rather than a settled question.

Regimen and dose are set by what progesterone is protecting against (endometrial protection vs. fertility support) — this is a starting reference, not a protocol.

Half-life & what it means for timing

Oral micronized progesterone’s elimination half-life is commonly cited around 5 hours, short enough that a once-daily dose produces a real peak — including the sedative allopregnanolone effect — a few hours after taking it. That’s the entire basis for the bedtime-dosing advice: taking it at night lets the sedation land during sleep instead of the middle of a workday.

Because the half-life is short and there’s no depot the way an oil-based ester provides, missing a dose matters more for progesterone than it does for a weekly testosterone shot — a skipped night in a cyclic regimen can undercut the whole point of the cycle, which is reliably shedding the uterine lining on schedule.

What to track & side effects

The most useful thing to track is simply whether the dose was taken and when, especially on a cyclic regimen where the 12–14-day count is what makes the regimen work — a missed or late dose is easy to lose track of without a log. For those on cream or other less-standardized routes, tracking symptoms (breast tenderness, mood, bleeding pattern) matters more, since a blood level is harder to interpret for that route.

Common side effects mirror the sedative mechanism: drowsiness, dizziness, and mild mood changes, most of the reason the bedtime-dosing convention exists. Oral micronized progesterone (Prometrium) is formulated in peanut oil, a well-known allergy consideration worth flagging to a prescriber. Bloating and breast tenderness are also commonly reported.

Frequently asked questions