Progesterone Medication-Driven Changes: What Lab Results Mean on HRT, Oral Progesterone, and Cycling Protocols

Q: Why can serum progesterone not tell me the uterine level with vaginal progesterone?

In a study of 14 postmenopausal women having hysterectomies, the median endometrial-to-serum ratio was 14.1 after vaginal gel versus 1.2 after intramuscular progesterone [2]. That means the endometrial concentration relative to serum was higher with the vaginal route in that study.

Q: How soon after an oral dose does progesterone peak?

Maximum concentrations were reached within 3 hours. Mean peak time was 1.5 hours at 100 mg and 2.3 hours at 200 mg after five daily doses in postmenopausal women [1]. Between-person variability was high [1].

Q: Does food change an oral progesterone level?

Yes. The label says food increased the bioavailability of oral micronized progesterone relative to fasting [1]. In a crossover study of 15 postmenopausal women, food raised absorption without shifting the time to peak [4].

Q: Why does the label say to take progesterone at bedtime?

The label states that progesterone may cause transient dizziness and drowsiness and should be taken as a single daily dose at bedtime [3][1]. The labeled indications are endometrial protection alongside conjugated estrogens and secondary amenorrhea [3].

By HealthRX.com Editorial Team

Published Jul 14, 2025Updated Nov 7, 2026Last reviewed Jul 14, 2025

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At a glance

Mean peak after 100 mg daily / 17.3 ng/mL [1]
Mean peak after 200 mg daily / 38.1 ng/mL [1]
Time to peak / within 3 hours [1]
Spread between women / wide; SD exceeds the mean [1]
Label dose, endometrial protection / 200 mg at bedtime, 12 days per 28-day cycle [3]
Endometrial-to-serum ratio / 14.1 vaginal vs 1.2 intramuscular [2]

What an Oral Progesterone Result Reflects

In postmenopausal women taking daily oral doses for five days, mean peak serum progesterone was 17.3 ng/mL at 100 mg, 38.1 ng/mL at 200 mg and 60.6 ng/mL at 300 mg, with mean peak times of 1.5, 2.3 and 1.7 hours [1]. Concentrations appeared linear and dose proportional from 100 to 300 mg per day, but the spread between women was large: at 100 mg the standard deviation was 21.9 ng/mL, larger than the mean [1]. Maximum concentration was reached within 3 hours of a dose [1]. Food increased bioavailability relative to fasting [1]. In a 15-woman crossover study of oral micronized progesterone (Utrogestan), food raised peak concentration without changing time to peak, and oral bioavailability was about 8.6% relative to intramuscular progesterone [4].

Vaginal and Intramuscular Progesterone

In a study of 14 postmenopausal women, progesterone was given as vaginal gel 90 mg or intramuscular progesterone 50 mg at 8:00 AM and 8:00 PM the day before surgery and again at 6:00 AM on the day of surgery. The median endometrial-to-serum ratio was 14.1 after vaginal gel and 1.2 after intramuscular progesterone (P<.005) [2]. This study does not establish a serum target for vaginal use.

Cycling Protocols and the Label

The Prometrium label says to give 200 mg orally as a single dose at bedtime for 12 days sequentially per 28-day cycle, to a postmenopausal woman with a uterus who is receiving daily conjugated estrogens tablets [3]. For secondary amenorrhea, the label says 400 mg at bedtime for 10 days [3]. In the label's 36-month endometrial trial of 358 women, hyperplasia occurred in 6% (7 of 117) on cyclic progesterone 200 mg plus conjugated estrogens 0.625 mg, 64% (74 of 115) on conjugated estrogens alone and 3% (3 of 116) on placebo [3]. The label's estrogen is conjugated estrogens, not estradiol [3]. The label states that Prometrium may cause transient dizziness and drowsiness and should be taken as a single daily dose at bedtime [1].

Frequently asked questions

Why can serum progesterone not tell me the uterine level with vaginal progesterone?

In a study of 14 postmenopausal women having hysterectomies, the median endometrial-to-serum ratio was 14.1 after vaginal gel versus 1.2 after intramuscular progesterone [2]. That means the endometrial concentration relative to serum was higher with the vaginal route in that study.

How soon after an oral dose does progesterone peak?

Maximum concentrations were reached within 3 hours. Mean peak time was 1.5 hours at 100 mg and 2.3 hours at 200 mg after five daily doses in postmenopausal women [1]. Between-person variability was high [1].

Does food change an oral progesterone level?

Yes. The label says food increased the bioavailability of oral micronized progesterone relative to fasting [1]. In a crossover study of 15 postmenopausal women, food raised absorption without shifting the time to peak [4].

Why does the label say to take progesterone at bedtime?

The label states that progesterone may cause transient dizziness and drowsiness and should be taken as a single daily dose at bedtime [3][1]. The labeled indications are endometrial protection alongside conjugated estrogens and secondary amenorrhea [3].

References

U.S. Food and Drug Administration. U.S. Food and Drug Administration
Cicinelli E, de Ziegler D, Bulletti C, Matteo MG, Schonauer LM, Galantino P. Direct transport of progesterone from vagina to uterus. Obstet Gynecol. 2000;95(3):403-6. Direct transport of progesterone from vagina to uterus
U.S. National Library of Medicine, DailyMed. Prometrium progesterone capsules (Acertis), Rev. 02/2026 prescribing information. Prometrium progesterone capsules (Acertis), Rev. 02/2026 label
Simon JA, Robinson DE, Andrews MC, Hildebrand JR, Rocci ML, Blake RE, et al. The absorption of oral micronized progesterone: the effect of food, dose proportionality, and comparison with intramuscular progesterone. Fertil Steril. 1993;60(1):26-33. The absorption of oral micronized progesterone: the effect of food, dose proportionality, and comparison with intramuscular progesterone

Evidence overview for Progesterone Medication-Driven Changes: What Lab Results Mean on HRT, Oral Progesterone, and Cycling Protocols