A 26-year-old woman attends her GP surgery with a positive home pregnancy test, confirmed on a urine test in clinic. She has taken a combined oral contraceptive pill (30 micrograms ethinylestradiol) every day for three years and has missed none, taking it at the same time each evening. She has had no vomiting or diarrhoea. Three weeks ago she finished a course of amoxicillin for sinusitis. Two months ago she began an over-the-counter herbal remedy for low mood. She has no other medical history and takes no other medicines.

What is the single most likely cause of this contraceptive failure?