Plan B works by giving the body a short, high, burst of synthetic hormones. This disrupts hormone patterns needed for pregnancy. It affects the ovaries and the development of the uterine lining, making pregnancy less likely. Depending upon where the woman is in her menstrual cycle, the EC hormones prevent pregnancy in different ways. It prevents ovulation (the egg leaving the ovary and moving into the fallopian tube). It blocks the hormones needed for the egg to be able to be fertilized. It may affect the lining of the uterus and alters sperm transport which prevents sperm from meeting the egg and fertilizing it.
The most common side effects include: nausea, abdominal pain, fatigue, headache, menstrual changes, dizziness, breast tenderness, vomiting, and diarrhea.
The drug, not to be confused with the RU-486 mifepristone pill, is effective as an emergency contraceptive if taken within 72 hours of unprotected sex or contraceptive failure, and is not intended – or effective – as an abortion pill. Needless to say, it should not be used to replace other forms of contraceptives.