progestin

A progestin is a synthetic stand-in for progesterone, the natural hormone that prepares and maintains the uterus for pregnancy. If estrogen sets the stage in the first half of the menstrual cycle, progesterone takes over in the second half to settle the uterine lining. Progestins are the laboratory versions designed to do the same job reliably as medicines.

Like natural progesterone, progestins act on the progesterone receptor, a nuclear receptor, changing gene expression in the uterus, breast, and brain. In contraception they thicken cervical mucus, thin the uterine lining, and, often together with estrogen, suppress ovulation. Many progestins are chemically related to either progesterone or testosterone, and these structural differences give individual agents (such as levonorgestrel, norethisterone, and drospirenone) slightly different side-effect tendencies, for example varying degrees of androgen-like or fluid-retaining activity.

Therapeutically progestins appear in combined and progestin-only contraceptive pills, implants, injections, and intrauterine systems, and they are added to estrogen in menopausal hormone therapy. That addition is not optional in women with a uterus: the progestin opposes estrogen's stimulation of the uterine lining, protecting against the overgrowth and cancer risk that unopposed estrogen would cause. Common nuisances include irregular bleeding, breast tenderness, and mood changes.