Letrozole is not chemotherapy, it is a type of hormone therapy that is used to treat people with breast cancer that is hormone receptor-positive. If your cancer is hormone receptor-negative, then letrozole will not be of any benefit. Traditional chemotherapy agents stop cancer cells from growing, dividing, and making more cells. Letrozole works by blocking the action of the enzyme aromatase, which prevents the body from converting androgens into estrogens. Estrogen is a hormone that causes estrogen receptor-positive (ER+) breast cancer to grow. Letrozole belongs to the class of medicines known as aromatase inhibitors.