Many forms of HRT contain the estrogen, 17-ß estradiol, which is identical to the natural female hormone. It is usually produced from plant materials rather than from any human or animal source. Conjugated equine estrogens are also widely used. These are extracted from the urine of pregnant mares. The estrogens used in HRT are very different than those used in oral contraceptives.

Natural progesterone is very poorly absorbed when taken as a tablet and therefore a number of synthetic alternatives (progestogens) have been developed. These all act to protect the lining of the womb.