Depending upon the time during the menstrual cycle that the emergency contraceptives are taken, these may prevent pregnancy by blocking the implantation of the fertilized egg in the uterus, by inhibiting ovulation, or by interfering with fertilization of the egg.
To be considered a possible candidate for emergency contraceptive pills a woman should take the medication within 72 hours of unprotected intercourse, although there is some evidence that they may be effective in the 5 days following intercourse.
Because the pills may be taken up to 72 hours later, the term "morning-after pill" is misleading.
However, the pills are most effective when taken as soon as possible after unprotected intercourse.
The only known contraindication to emergency contraception is pregnancy, so a woman must not be pregnant when these methods are used.
Emergency hormonal contraception may be taken on any day of the menstrual cycle.