What is letrozole, how does it work and what are the side effects? Our Clinical Nurse Specialist Louise takes us through some information about letrozole. Letrozole is a drug used to treat breast cancer in women who have gone through the menopause and whose breast cancer is oestrogen receptor positive. You may also hear it called Femara, which is its best-known brand name. There are a number of other brands of letrozole, all of which contain the same dose of the drug. Some breast cancers use oes