Sita is a 22 year old who presents after an episode of unprotected sexual intercourse (UPSI) 4 days ago. She is not in a stable relationship and is requesting emergency contraception. Her last menstrual period was 1 week ago and her cycles are regular. She is a university student and smokes 15 cigarettes per day. After discussion, she has agreed to a prescription of Ulipristal Acetate.
Which one of the following statements is least correct regarding emergency contraception?