Femara is a medication that is used in the treatment of certain breast cancers. It’s most commonly used for hormone-receptor-positive breast cancer. The active ingredient is letrozole. Letrozole is known as an aromatase inhibitor, also called an anti oestrogen, because it helps reduce the amount of oestrogen produced in the body. Femara is a pill that you take by mouth. Depending on your medical condition and treatment response, the dosage will vary.