The morning after pill is basically a high dose birth control pill. The exact mechanism of how the pill works is uncertain at this time. Emergency contraceptives may act by inhibiting ovulation, interfering with transport, preventing the actual implantation, or by causing regression of the corpus luteum
Side effects include:
Most common are nausea and vomiting
Uncommon side effects include: irregular bleeding, dizziness, fatigue, headache, breast tenderness, and lower abdominal pain.