A 35-year-old woman on long-term phenytoin therapy for epilepsy presents requesting contraceptive advice. She is currently using a combined oral contraceptive (COC) containing 35 µg ethinyl estradiol and norethisterone. Her last menstrual period was 6 weeks ago, and a urine pregnancy test is negative. She reports breakthrough bleeding and irregular periods over the past 3 months. What is the most appropriate next step in management?