Biological activity of oral contraceptives.

The synthetic steroid hormones used in oral contraceptives differ in their effects and potencies. Because all low-dose preparations currently sold in the United States contain the same estrogen, the differences among progestin components are of clinical significance. Synthetic progestins may have progestational, estrogenic, antiestrogenic, and androgenic effects; all have antiovulatory effects. Means for measuring the different effects of steroid hormones, ranging from animal assays to human studies, are reviewed. The problems of generalizing from effects of one steroid in an animal system to combination steroids in the human system are emphasized. Our evolving concept of desirable and undesirable effects of progestins is reviewed, the concept of minimal effective dose introduced, and a conclusion suggested from the limited human data available. A plan for the selection of contraceptive steroids for human use is proposed.