The pill works by eroding the lining around your vagina, making it near impossible for a fertilized egg to attach. Eventually it will run out from either being ejected when peeing, or from the lining shedding during your period. Some will be absorbed by the lining and mucus on the vagina wall. Some will swim around looking for your egg to fertalize for up to 7 or so days. Some of the sperm will die right away from the acidity of the vagina. A lot of it will flow back out afterward.
