Pharmacokinetics of oral contraceptive steroids after morning or evening administration.

An oral contraceptive containing ethynyloestradiol and norethisterone was administered to six women in the morning and in the evening using a cross-over design. Serum levels of ethynyloestradiol and norethisterone were measured at various times after administration. There was no significant difference in a number of pharmacokinetic parameters between the two times of administration, suggesting that morning or evening administration of the contraceptive are equally effective.