Karen is a 19 year old university student who attends the GP surgery requesting emergency contraception. She had unprotected sexual intercourse 3 days ago with a new partner. Her last menstrual period was 6 days ago. She agrees that a sexually transmitted infection screen is sensible and is keen to have Ulipristal Acetate as she had this last year with no problems. She does not want a coil fitted.
Which one of the following statements is least correct regarding emergency contraception?