Most forms of HRT contain a synthetic type of progesterone called a progestogen. Progestogens do not have any effect on menopausal symptoms. They are important in HRT because they have a protective effect on the lining of the womb (endometrium). If estrogens are given alone (known as unopposed therapy) to women who have not had a hysterectomy, they can cause overgrowth of the endometrium which carries a risk of endometrial cancer. Giving a progestogen with the estrogen greatly reduces this risk.