Absorption of oral progesterone is influenced by vehicle and particle size.

The oral route of progesterone administration has long been considered impractical because of poor absorption and short biologic half-life. Recent reports suggest that micronization of progesterone enhances absorption and increases serum and tissue levels of progesterone. This study checks serum progesterone levels before and 0.5, 1, 2, 3, 4, and 6 hours after oral administration of 200 mg of progesterone in seven subjects. Progesterone was plain milled, micronized, plain milled in oil, micronized in oil, or micronized in enteric-coated capsules. All patients exhibited a significant increase in serum progesterone levels after oral progesterone administration. Mean peak progesterone levels (30.3 +/- 7.0 ng/ml) (p less than 0.005) were achieved with micronized progesterone in oil at 2.0 +/- 0.3 (p less than 0.05) hours after administration. Four types of oral progesterone had equivalent mean peak elevations and mean times to peak: plain milled, 9.6 +/- 2.5 ng/ml at 4.0 +/- 0.5 hours; micronized 13.2 +/- 2.4 ng/ml at 3.2 +/- 0.4 hours; plain milled in oil, 11.3 +/- 3.0 ng/ml at 4.0 +/- 0.5 hours; and micronized in enteric-coated capsules, 11.2 +/- 3.0 ng/ml at 4.1 +/- 0.7 hours. Contrary to traditional teaching, these data show that significant serum progesterone levels can be achieved by oral administration. Absorption can be significantly improved by the physical characteristics of the progesterone and the vehicle used with oral administration.