Emergency contraceptive pills are effective when taken once, but only for this sexual intercourse. The main ingredient of the emergency contraceptive pill is levonorgestrel, which mainly delays and stops the release of egg cells, thus playing the role of contraception, this method of contraception is more effective, and the sooner you take the drug after sex, the better the effect of contraception. However, the emergency contraceptive pill is only effective for this sexual intercourse, if you have sex again after using the pill, this situation has lost the effect of the drug, there is still a possibility of pregnancy. Levonorgestrel may cause some adverse reactions, such as menstrual changes, nausea, vomiting, breast tenderness, dizziness, headache symptoms, and in some cases, abnormal uterine bleeding. It is contraindicated in known or suspected pregnancy, breast cancer, cancer of reproductive organs, and abnormal liver function. Emergency contraceptive pills are recommended to be taken under the guidance of a doctor, and should not be used blindly on their own to avoid adverse effects.