They are classified according to their structure in C19 and C21 progestagens. The C19 ones are derived from testosterone, the C21 ones from progesterone. C21 progestagens include cyproterone acetate, dydrogesterone, medroxyprogesterone acetate, chlormadinone acetate, megestrol and promegestone. Of these, dydrogesterone is structurally most similar to progesterone. C19 progestagens include norethisterone, (levo)norgestrel, lynestrenol, desogestrel, norgestimate, gestodene and tibolone.
Progestagens differ in their potency (affinity for progesterone receptors) and side-effects. Such side-effects may be androgenic (medroxyprogesterone and most C19 progestagens), antiandrogenic (cyproterone acetate), estrogenic, glucocorticoid (some C21 progestagens) or antimineralocorticoid (progesterone).
Most progestagens are used for their antiestrogenic properties in oral contraceptives to avoid overstimulation of the endometrium which could lead to endometriosis. Medroxyprogesterone acetate (brand name Depo-Provera) is used in birth control. Cyproterone acetate is used mostly as an antiandrogen.