Comparative bioavailability of orally and vaginally administered progesterone.

Objective: To study the pharmacokinetics of progesterone (P) in healthy premenopausal female volunteers to compare the bioavailability of orally or vaginally administered hormone.
Design: Subjects were randomly allocated to receive either oral P or a vaginal pessary then crossed over to the alternate preparation 1 month later.
Setting: The study was conducted in outpatient setting.
Subjects: All subjects were healthy, normal female volunteers who underwent a physical and gynecological examination before the study. None were using oral contraceptives. Ten subjects (mean age 32.6 +/- 7.3 years) entered the study and all completed it.
Interventions: Progesterone was administered as 200 mg of micronized hormone or as a pessary containing 400 mg.
Main Outcome Measure: Plasma levels of P were measured by radioimmunoassay to test the apriori hypothesis of similar bioavailability.
Results: Peak plasma P concentrations attained within 4 hours after oral administration ranged from 8.5 to 70.6 ng/mL, whereas after vaginal administration the peak levels were attained within 8 hours and ranged from 4.4 to 181.1 ng/mL. Considerable interindividual variation was noted. Area under the plasma concentration-time curve for the two formulations was not significantly different (F = 1.09; P greater than 0.1; ANOVA).
Conclusions: The two formulations had similar bioavailability.