Letrozole chemotherapy drug. Computer model of a molecule of the chemotherapy drug letrozole. This drug, which is marketed as Femara, is used to treat breast cancer. It belongs to a class of drugs called aromatase inhibitors. These drugs stop the production of oestrogen, a hormone responsible for the growth and recurrence of many breast cancers in post-menopausal women. Letrozole is also used in fertility treatment to induce ovulation. The molecule's chemical formula is C17.H11.N5. The atoms are shown as colour-coded spheres in this space-filled model: carbon (light blue), hydrogen (yellow), and nitrogen (dark blue).