The emergency contraceptive pill is useful after sex, but the short-acting contraceptive pill is ineffective. Because the emergency contraceptive pill contains a large dose of progesterone, so much progesterone into the female body will inhibit female ovulation, that is, if a woman has not yet ovulated, after taking it will prevent her ovulation. If you have already ovulated, after taking it will affect the peristalsis of the fertilized egg, will also affect the changes of the uterine lining, which can prevent the fertilized egg in the uterus inside the bed, so after having sex to take emergency contraceptive pills have the effect. The short-acting contraceptive pill contains a relatively low component of the drug, this time to eat is not much effect, so it is not recommended to eat.