Other, less common, side effects are breast tenderness, headaches, menstrual irregularities, and dizziness. The FDA has declared both combined EC and progestin-only EC to be safe and effective. If 100 fertile women engaged in unprotected sex in the second or third week of their menstrual cycles and did not use EC, eight would become pregnant. But if they used Preven, only two would become pregnant, and if they used Plan B, only one would become pregnant. It is recommended that EC pills be taken within 72 hours after unprotected sex.
Studies show that they are most effective when taken as soon as possible.
EC is safe and has not been known to cause serious health problems for any women, including those who would otherwise not be able to take birth control pills on a regular basis. EC pills may work in a variety of ways. If a woman is already pregnant when she takes EC pills, they will not harm the fetus. An IUD can prevent sperm from fertilizing an egg and can inhibit implantation of a fertilized egg as well. Emergency contraception is contraception, not abortion.