Ulipristal acetate is a selective progesterone receptor modulator. It acts by blocking the receptor of a hormone in the body called progesterone, which is involved in controlling the growth of the lining of the womb. In some women, progesterone may promote the growth of fibroids, which may cause symptoms such as heavy uterine bleeding, anaemia and abdominal pain. When progesterone activity is blocked, fibroid cells stop dividing and eventually die which reduces the size of the fibroids and reduces the symptoms caused by them.