ellaOne contains 30mg Ulipristal acetate.
It is thought to work by preventing or delaying the egg being released by the ovary. If the embryo has already implanted, then it does not affect this. It will therefore not terminate a pregnancy if it has already occurred.
Studies have shown if 1,000 women had unprotected sex once, about 60 to 80 would become pregnant. If all those women had taken ulipristal acetate, about 9 to 18 would have become pregnant. It will, however, depend on when in the cycle it was taken.
If ovulation has already taken place, then it will not be effective. If you have a 28-day cycle, then you are likely to ovulate after 14 days from the first day of your period. Ovulation and menstrual cycles can be unpredictable so it may be difficult to work out your day of ovulation.
If you think you may have already ovulated, then you should consider the IUCD. Contact your GP surgery or family planning clinic to arrange this as soon as possible. You can also contact NHS 111 for further advice.
If you have unprotected sex at any time after taking the emergency pill, you can become pregnant. It does not continue to protect you against pregnancy after you have taken it.
Using emergency contraception does not replace the necessary precautions required to prevent sexually transmitted diseases.