Aromatase inhibitors (AIs) are drugs that lower estrogen levels by stopping an enzyme in fat tissue (called aromatase) from changing other hormones into estrogen. (Estrogen can fuel the growth of breast cancer cells.) They don’t stop the ovaries from making estrogen. Aromatase inhibitors are used mostly in postmenopausal women, because the ovaries in premenopausal women produce too much aromatase for the inhibitors to block effectively. However, they can be used in premenopausal women in combination with drugs that suppress (or limit) ovarian function.

Sources:

American Cancer Society, 2021. Aromatase Inhibitors for Lowering Cancer Risk

National Cancer Institute, 2022. Hormone THerapy for Breast Cancer

Susan G. Komen, 2024. Aromatase Inhibitors.