The early onset of menstruation after taking the emergency contraceptive pill is due to the effect of the pill on the normal endocrine function of the ovaries. It also acts on the endometrium, causing the endometrium to peel and bleed early. The effect on menstruation after taking the emergency contraceptive pill has a lot to do with the time of taking the emergency contraceptive pill. First, if the time of taking the drug is in the first half of the menstrual cycle, the effect on the menstrual cycle after taking the drug is more likely to lead to early menstruation. If the time of taking the drug is around the 20th day of the menstrual cycle, it will not affect the menstrual cycle significantly. Third, if the time of taking the drug has almost reached the time of menstruation, it will lead to the delay of menstruation.