Sally is a 22 year old lady who is requesting emergency contraception following an episode of unprotected sexual intercourse (UPSI) with her boyfriend 2 days ago. Past medical history includes Epilepsy for which she takes Carbamazepine. She has a regular cycle and is currently on day 14 of her cycle (her usual cycle length is 28 days).

Which one of the following statements about emergency contraception in this scenario is least correct?