Letrozole

Key facts

• Letrozole (sold as Femara) is an aromatase inhibitor: it blocks the enzyme that makes estrogen in fat and other tissues, sharply lowering estrogen levels after menopause.
• It's taken as one tablet a day, usually for around 5 years, as treatment for hormone-receptor-positive breast cancer.
• Letrozole speeds up bone loss and raises fracture risk; expect bone density (DEXA) scans before treatment and periodically during it.
• Seek urgent care for chest pain, sudden shortness of breath, or signs of a stroke such as facial drooping, arm weakness, or slurred speech.

What letrozole treats

Letrozole treats hormone-receptor-positive breast cancer in postmenopausal women, both as initial treatment for early-stage disease and for cancer that has spread. It's also used to reduce the chance that early breast cancer returns after surgery, often following a course of tamoxifen. It only works reliably after menopause, when aromatase in fat and other tissues becomes the body's main remaining source of estrogen.

How letrozole works

After menopause the ovaries stop making estrogen, but an enzyme called aromatase still converts other hormones into small amounts of estrogen in fat, muscle, and other tissues. Many breast cancers depend on this estrogen to grow. Letrozole blocks aromatase, cutting circulating estrogen to very low levels and starving hormone-sensitive cancer cells of the signal they need to divide.

Before you take it

• Letrozole is for postmenopausal women only; it does not work reliably before menopause and must not be used in pregnancy, which it can harm.
• Tell your doctor about osteoporosis, previous fractures, or heart disease; low estrogen thins bone and can affect cholesterol and cardiovascular risk.
• Expect a bone density scan before starting and periodically afterward; your doctor may recommend calcium, vitamin D, or a bone-protecting medicine.
• Mention other hormone treatments, including tamoxifen, since combining hormone therapies is not usually appropriate.

Side effects

Common effects include hot flushes, joint and muscle aches or stiffness, fatigue, and mild nausea.

Stop and seek urgent medical care for any of these:

• New bone pain or a fall that could signal a fracture.
• Chest pain, sudden shortness of breath, or a fast or irregular heartbeat.
• Sudden weakness, numbness, or difficulty speaking.
• Severe swelling of the legs.

Safety essentials

• Letrozole's defining risk is accelerated bone loss: fracture risk rises during treatment, so bone density scans before and during treatment are a required part of care, not an optional extra.
• Take calcium and vitamin D if your doctor recommends them, and keep up weight-bearing exercise to help protect bone.
• Have cholesterol and cardiovascular risk reviewed periodically, since long-term low estrogen can affect both.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.