Plan B One-Step works with one dose.
Side effects may include nausea, vomiting, abdominal pain, fatigue, dizziness, menstrual changes and headache. According to the manufacturer, the morning-after pill is more than 80 percent effective in preventing pregnancy after a single act of unprotected sex.
Morning-after pills aren't the same as mifepristone (Mifeprex), the so-called abortion pill. Emergency contraceptive pills such as Plan B prevent pregnancy.