Hormones are substances produced naturally in the body. They act as chemical messengers and help control the activity of cells and organs. Hormonal therapies are drugs that interfere with the way hormones are made or how they work in the body.
Many breast cancers rely on the hormone oestrogen to grow. This type of breast cancer is called oestrogen receptor-positive (ER positive) breast cancer.
In women, after the menopause, oestrogen is no longer made in the ovaries. Instead, it is made in the fatty tissues of the body. This happens when an enzyme (a type of protein) called aromatase changes other hormones into oestrogen. Letrozole is a type of drug called an aromatase inhibitor. It blocks (inhibits) the activity of aromatase and reduces the amount of oestrogen in the body.