ProgestinsThe major portion of the contraceptive effect in hormonal methods is due to the progestin compound. Progestins alone confer the contraceptive benefits, while estrogens, for the most part, serve to regulate bleeding. Progestins in oral contraceptives include are derived from 19 nor-testosterone and include norethindrone, norethindrone acetate, ethynodiol diacetate, norgestrel, levonorgestrel, norethynodrel, desogestrel, norgestimate, and gestodene. The last three are sometimes called the 'new progestins' or the 'third generation' progestins; however, this label is of no scientific or clinical value and was developed for marketing purposes. Progestins should be categorized according to their active structure and their parent compound. There are three molecularly distinct types of progestins: estranes, gonanes, and pregnanes. Estranes include norethindrone, norethindrone acetate, ethynodiol diacetate, and lynestrenol. Gonanes include desogestrel, norgestimate, and gestodene. Gonanes and estranes differ in their half-life and with respect to their estrogenic and anti-estrogenic effects. Drospirenone is a spironolactone analog with anti-mineralocorticoid and anti-androgenic activity. Pregnanes are used in inject able methods. Table 1 provides a summary of the progestins currently used as contraceptives. Table 2 provides a summary of the combined oral contraceptive preparations currently on the market in the USA.

In contrast to the long list of progestin formulations, only two estrogenic compounds are used in hormonal contraceptives: ethinyl estradiol (EE) and mestranol. EE is pharmacologically active whereas mestranol must be converted into EE before it becomes active. Contraceptives currently on the market contain 35 micrograms of estrogen or less. Ethinyl estradiol is absorbed rapidly and undergoes extensive hepatic first pass metabolism. Its plasma half-life has been reported to be in the range of 10-27 hours. Its half-life in tissue, such as endometrium, appears to be longer.