Absorption and metabolism of oral progesterone when administered twice daily

The absorption, metabolism, and clearance of progesterone (P) from the peripheral circulation were investigated in five postmenopausal women after oral administration of 100 mg at 9:00 A.M. and 200 mg at 9:00 P.M. for 5 consecutive days. Mean peak plasma concentrations of P were observed 2 hours after ingestion of both the 100 and 200 mg doses and were 22.7 and 47.7 nmol/l, respectively. Of the three metabolites studied, the plasma concentrations of pregnanediol-3 alpha-glucuronide were most raised by treatment; those of 17-hydroxyprogesterone were least raised. Increases in the plasma levels of 20 alpha-dihydroprogesterone were more sustained than those of P, and the plasma concentrations remained elevated at approximately 20 nmol/l for at least 12 hours after P administration. We conclude that administration of oral P 100 mg in the morning and 200 mg at night increases the circulating concentrations of P and the biologically active metabolite 20 alpha-dihydroprogesterone, and that the duration of these increases is sufficient to evoke progestational responses in responsive end-organs.