Metabolism of D- and L-norgestrel in humans.

Levels of DL-norgestrel, D-norgestrel, L-norgestrel and norethisterone were measured in blood by radioimmunoassay after oral administration of the progestin to 3 healthy male subjects and the half-lives calculated. Half-lives for D-norgestrel were much lower than those of L-norgestrel and similar to those of norethisterone. Measurement of norgestrel in blood can therefore be misleading unless the biologically active D-isomer is estimated. Since there was no difference in half-lives between norgestrel and norethisterone the greater potency of the former is probably due to it more strongly binding to target organ receptors.