Plan B: Brand name for a progestin-only emergency contraceptive designed to prevent pregnancy within 72 hours after a contraceptive accident or unprotected sex. However, early treatment is significantly more effective. Each 12 hours of delay in the start of treatment after intercourse generally reduces efficacy by 50%. Levonorgestrel, the active ingredient, is believed to act principally by interfering with the process of ovulation. It may also interfere with fertilization or implantation. It is not effective once implantation has begun. It is not an abortifacient. Common side effects include nausea, abdominal pain, fatigue, headache, and menstrual changes.