Moderate
Carbamazepine × Estradiol
Antiepileptics. Carboxamide derivatives×Estrogens — natural and semisynthetic
Mechanism
Carbamazepine is a strong CYP3A4 inducer. Plasma oestradiol falls by 40–60%. On menopausal hormone therapy, menopausal symptoms may return; on combined oral contraceptives, unplanned pregnancy risk arises.
Symptoms
Return of vasomotor symptoms: hot flushes, night sweats, sleep disturbance. On oral contraceptives: breakthrough bleeding, conception risk.
Management
On menopausal hormone therapy, double the oestradiol dose or switch to transdermal forms (patch, gel — minimal hepatic metabolism). For contraception on carbamazepine, combined oral contraceptives are unsuitable — choose a levonorgestrel IUS, copper IUD, or depot medroxyprogesterone acetate.