Letrozole is a hormone-therapy tablet for breast cancer that is hormone-receptor-positive — meaning the cancer cells grow in response to oestrogen — in women who have been through the menopause. It is used after surgery to reduce the chance of the cancer coming back (adjuvant treatment, usually for five years, sometimes longer), for advanced or metastatic disease, often with a CDK4/6 inhibitor such as palbociclib, and sometimes before surgery to shrink a tumour.
After the menopause the ovaries stop making oestrogen, but the body still produces small amounts from other hormones using an enzyme called aromatase. Letrozole blocks that enzyme almost completely, starving the cancer of oestrogen. It is not effective on its own in women who are still having periods, because the ovaries respond by making more.
Letrozole is a small tablet taken once a day with or without food, and it is inexpensive. Hot flushes, joint aches and bone thinning are the main side effects. Outside cancer it is also used, off-label, to stimulate ovulation in fertility treatment. It is a prescription-only medicine; a valid prescription must be provided at checkout.