Letrozole (Femara): breast cancer treatment and fertility use

Letrozole is a prescription-only tablet. The NHS lists treatment of hormone-dependent breast cancer in postmenopausal women, prevention of recurrence after primary treatment, prevention in high-risk postmenopausal people, use when surgery is not possible or to shrink a tumour before surgery, and occasional use in men or premenopausal women with breast cancer. Femara is the brand name.

It is also sometimes used as a fertility treatment for polyendocrine metabolic ovarian syndrome (PMOS), which the NHS describes as a rare use that fertility specialists oversee.

Letrozole belongs to the aromatase inhibitors. They reduce the amount of aromatase in the body, and this stops the body producing oestrogen. Some breast cancers use oestrogen to grow, so lowering it slows or prevents them.

Oestrogen falls straight away, but the NHS says it takes several weeks or months for the medicine to work fully, and side effects usually improve after a few months.

Most adults aged 18 and over can take letrozole. Raise the following with the prescriber before starting:

• An earlier allergic reaction to letrozole or another medicine.

• Still having periods.

• Pregnancy or breastfeeding.

• A wish to become pregnant, other than under a prescribed fertility plan.

• Serious kidney or liver disease.

• Osteoporosis, meaning fragile or brittle bones.

The tablets are 2.5mg and the usual dose is one tablet once a day, at the same time each day. Swallow whole with water, with or without food, and do not crush or chew. Treatment usually lasts about 5 years and sometimes up to 10.

If your next dose is due within 3 hours, skip the missed one; otherwise take it when you remember. Never double up. Setting an alarm can help. Do not stop, or take a break, without your specialist's agreement. If you take too much and feel unwell, call NHS 111; symptoms can include nausea, blurred vision or a rapid heartbeat.

More than 1 in 100 people have hot flushes and sweating, vaginal bleeding (usually in the first weeks), tiredness, nausea or vomiting, loss of appetite, mild muscle or bone aches, hair thinning and low mood. Cooler rooms, small frequent meals, gentle exercise, a regular sleep schedule and cutting back on spicy food, caffeine, smoking and alcohol can ease some of these.

• Call 111 or see a doctor for a throbbing, swollen, warm, red limb that could be a clot, muscle weakness or joint or tendon swelling, fever or chills, frequent mouth ulcers or sore throat, yellow skin or eyes, urinary problems or blurred vision.

• Call 999 for sudden weakness, a drooping face or slurred speech, sudden severe chest pain, or signs of anaphylaxis.

• Report suspected side effects through the Yellow Card scheme.

Because oestrogen helps keep bones strong, long-term use can cause osteoporosis. The NHS says bone density scans are recommended before treatment, at 1 to 2 years and after treatment, and bisphosphonate medicines may be offered to protect bone. Blood pressure and cholesterol can rise, so they are checked regularly.

Letrozole can change appetite, and for some people weight changes; a dietitian referral is available if weight gain becomes a problem.

Letrozole is not recommended in pregnancy or breastfeeding because it affects hormones. If pregnancy is possible, speak to a doctor urgently. Fertility specialists can answer questions about its use in ovulation treatment.

It should not be taken with menopause medicines such as hormone replacement therapy, nor with herbal remedies for menopause symptoms, because they can stop letrozole working as well as it should. There is no evidence that alcohol causes problems, though some people notice hot flushes or redness. Avoid driving if you feel extremely tired, dizzy or have blurred vision.