Emergency contraceptive pills can be used within 72 hours of unprotected sex, the sooner the pill is taken, the better the effect, if more than 72 hours before taking it is not effective. Emergency contraceptive pills are highly effective progestins, eat mainly inhibit the hypothalamus pituitary ovarian axis, also inhibit ovulation, and change the normal environment of the uterine cavity, to be able to play a contraceptive effect. This drug can not be taken more than 3 times a year, only 1 time a month, eat more will cause endocrine disorders, ovulation abnormalities, irregular vaginal bleeding. The success rate of this kind of drug contraception can only reach 85%-90%, there are more than 10% of the people who take the drug will also be pregnant, so come to the time of menstruation, if it does not come to the hospital in time to check, to rule out unwanted pregnancy.