Fentanyl is a kind of estrogen and progestin preparation. There are the following reasons why women do not get menstruation after taking Fentanyl: in the first case, women have thin endometrium or uterine adhesions, and Fentanyl does not make the endometrium grow and thicken as expected, so it does not lead to the onset of menstruation. In the second case, if a woman stops taking Fentanyl for too short a time, the endometrium does not grow as expected and the progesterone level is high and has not withdrawn yet, it may also lead to no menstruation. In the third case, if a woman has a normal sexual life during the course of taking Fentanyl and does not use contraception, she may also become pregnant and therefore will not have her period. In this case, it is better for women to go to the hospital for gynecological ultrasound and blood HCG measurement about a week after stopping the medication. If pregnancy does not occur and the endometrium is thick, progestin can be given to promote menstruation.