Estrogen + progestin suppress the hypothalamic-pituitary axis (↓ FSH/LH) → no LH surge → ovulation inhibited; progestin also thickens cervical mucus and thins the endometrium.
Black Box Warning
Cigarette smoking increases the risk of serious cardiovascular events from combined oral contraceptives; the risk rises with age (especially over 35) and with the number of cigarettes smoked — contraindicated in women over 35 who smoke.
Clinical Notes
Some products are also indicated for heavy menstrual bleeding or moderate acne; PCOS is not an approved indication. Contraindicated with prior VTE, smoking over 35, and headache with focal neurologic symptoms or migraine with or without aura over 35.