Plasma levels of progesterone after vaginal, rectal, or intramuscular administration of progesterone

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Plasma levels of progesterone were determined after vaginal, rectal, or intramuscular administration of 10 to 100 mg. of progesterone to 35 female volunteers. Progesterone was assayed by a competitive protein-binding technique. The absorption was very rapid by all three routes of administration, usually resulting in peak plasma levels of progesterone within the first 8 hr. after administration. The plasma levels remained elevated for longer periods of time than would be expected from the rapid rate of disappearance described for progesterone. Plasma levels corresponding to those encountered during the luteal phase of the menstrual cycle were attained with an intramuscular injection of 25 mg. of progesterone in oil or four times this dose by vaginal or rectal administration, while an intramuscular dose of 100 mg. resulted in a mean peak level corresponding to mid-pregnancy plasma levels of progesterone.