Pharmacology of contraceptive steroids: a brief review.
Blood levels from a given oral dose of medication vary widely from patient to patient, even within a relatively homogeneous population, and from group to group across national borders. An important mechanism of inter-individual variation is the first pass through the enterohepatic circulation. When drugs such as ethinyl estradiol and norethindrone undergo this process, part of an orally administered dose is inactivated, which results in wide ranges of bioavailability. In connection with this process, these drugs are also susceptible to various drug-to-drug interactions. Levonorgestrel is not subject to first-pass extraction by the liver, which results in virtually 100% bioavailability. It is unclear whether interpopulation variations in blood levels result from ethnic, geographic, nutritional, or other phenomena. Hepatic mechanisms are qualitatively different as demonstrated by striking differences in urinary patterns of ethinyl estradiol conjugates and hydroxylations of ethinyl estradiol from country to country.
Goldzieher
J W
JW
Department of Obstetrics and Gynecology, Baylor College of Medicine.
D001682:Biological Availability; D003276:Contraceptives, Oral / Q000493:pharmacokinetics* / Q000494:pharmacology; D004347:Drug Interactions; D004997:Ethinyl Estradiol / Q000493:pharmacokinetics; D006801:Humans; D016912:Levonorgestrel; D009640:Norethindrone / Q000493:pharmacokinetics; D009644:Norgestrel / Q000493:pharmacokinetics
D003276:Contraceptives, Oral; D009644:Norgestrel; D004997:Ethinyl Estradiol; D016912:Levonorgestrel; D009640:Norethindrone