Letrozole (Femara) | Cancer Research UK
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What letrozole is

Letrozole is a type of hormone therapy drug used to treat breast cancer in women who have had their menopause. It is also called Femara.


How letrozole works

Letrozole lowers the levels of the female sex hormone oestrogen in the body. Oestrogen stimulates some breast cancers to grow. These breast cancers are called hormone sensitive or hormone receptor positive. Letrozole can stop or slow the growth of these cancers. It can also reduce the chance of a breast cancer coming back after surgery.

Letrozole only works in women who have had their menopause. It is a type of drug called an aromatase inhibitor. After the menopause, women don’t produce oestrogen from their ovaries. But they still produce a small amount by using an enzyme called aromatase which turns other sex hormones called androgens into oestrogen. This change happens mainly in fatty tissue, muscle and the skin. Aromatase inhibitors block aromatase so that it can’t change the androgen into oestrogen.


How you have letrozole

You take letrozole as a tablet, once a day. It is best to take the tablets at the same time each day. You can take them with or without food.

It is very important that you take tablets according to the instructions your doctor or pharmacist gave you. You should take the right dose, not more or less. And never stop taking a cancer drug without talking to your specialist first.

If you accidentally take too much Femara, contact a doctor or hospital straight away. You may need medical treatment. 

If you forget to take a dose and it is almost time for your next dose (for example, within 2 or 3 hours), skip the dose you missed and take your next dose when it is due. Otherwise, take the dose as soon as you remember, and then take the next dose at the normal time. Don't take a double dose.


Tests during treatment

You have blood tests before starting treatment and regularly during your treatment. The tests check your levels of blood cells and other substances in the blood. They also check how well your liver and kidneys are working.


About side effects

We've listed the side effects associated with letrozole. You can use the links to find out more about each side effect. Where there is no link, please go to our information about cancer drug side effects or use the search box at the top of the page.

You may have a few side effects. They may be mild or more severe. A side effect may get better or worse through your course of treatment. Or more side effects may develop as the course goes on. This depends on

  • How many times you've had the drug before
  • Your general health
  • The amount of the drug you have (the dose)

The side effects may be different if you are having letrozole with other medicines.

Tell your doctor or nurse straight away if any of the side effects get severe.


Common side effects

More than 1 in every 10 women have one or more of these effects.

  • Hot flushes and sweats – this happens in about 3 out of 10 women (30%)
  • Pain in joints or bones – this affects about 2 out of 10 women (20%)
  • Tiredness and weakness (fatigue) occurs in about 2 out of 10 women (20%)
  • Increased levels of cholesterol in the blood – this is usually only slightly increased

Occasional side effects

Between 1 and 10 in every 100 people have one or more of these.

  • Skin rashes that are usually mild – let your doctor or nurse know if you have a rash or dry skin
  • Headaches affect about 1 in 10 women (10%)
  • Dizziness occurs in around 1 in 10 women (10%) – don't drive or operate machinery if you have this
  • Generally feeling ill (malaise)
  • Feeling or being sick – this is usually mild and can be easily controlled by anti sickness tablets. It affects about 1 in 10 women (10%)
  • Fluid retention causing ankle and or finger swelling occurs in about 1 in 10 women (10%)
  • Loss of appetite
  • Indigestion
  • Hair thinning
  • Diarrhoea – if this happens it is usually mild. Drink plenty of fluids. If it gets severe or continues you could become dehydrated, so tell your doctor or nurse
  • Constipation occurs in some people. Drink plenty of fluids. Tell your doctor or nurse if it gets severe or continues for more than 3 days
  • Vaginal dryness – let your doctor or nurse know if you have this. Medicines can help
  • Lower interest in sex (reduced libido)
  • Sadness or depression
  • A cough and breathlessness – this affects less than 1 in 10 women (10%)
  • Weakening of the bones (osteoporosis) caused by a lack of oestrogen over a long period of time – your bones may be more likely to break. You will have a DEXA scan to check your bone strength before you start treatment
  • Vaginal bleeding – this mainly happens when women change from one type of hormone therapy to another during the first few weeks of treatment. Tell your doctor or nurse if the bleeding continues. This affects fewer than 5 out of 100 women (5%)
  • Muscle pain
  • Weight gain – it can be difficult to keep a normal weight when taking hormone therapy. Talk to your nurse about eating healthily and ways to maintain a healthy weight
  • Higher blood pressure than normal (hypertension) – you will have regular blood pressure checks
  • Pain in the abdomen – tell your doctor or nurse if you have this

Rare side effects

Fewer than 1 in 100 people have these effects.

  • Nervous disorders, such as anxiety, nervousness, feeling irritable, drowsiness, or memory problems
  • Difficulty sleeping
  • Changes in sensation, especially touch
  • Eyesight changes such as blurred vision
  • Red, sore eyes
  • A faster heart rate or feeling of the heart beating (palpitations)
  • Joint stiffness (arthritis)
  • Pain, stiffness, and clicking in a finger or thumb and a small lump in the palm at the base of the affected finger or thumb. This condition is called trigger finger and the affected finger may get stuck when bent towards the palm. Tell your doctor or nurse if you have this
  • Pain, a weaker grip, and numbness and tingling can occur in one or both hands, particularly in the fingers and thumb. This condition is called carpal tunnel syndrome. It is caused by pressure on a nerve that passes through the wrist into the hand. Tell your doctor or nurse if you have this
  • Breast pain
  • A high temperature (fever)
  • Taste changes
  • A dry mouth and feeling thirsty
  • Weight loss
  • Urine infections

Important points to remember

Talk to your doctor, pharmacist or nurse about all your side effects so they can help you manage them. They can give you advice or reassure you. Your nurse will give you a contact number to ring if you have any questions or problems. If in doubt, call them.

Other medicines

Tell your doctor about any other medicines you are taking, including vitamins, herbal supplements and over the counter remedies. Some drugs can react together.

Pregnancy and contraception

This drug may harm a baby developing in the womb. It is important not to become pregnant while you are having treatment and for a few months afterwards. Even if you have had your menopause, talk to your doctor or nurse about effective contraception before starting treatment.


More information on letrozole

This page doesn't list all the very rare side effects of this treatment that are very unlikely to affect you. For further information look at the Electronic Medicines Compendium website at www.medicines.org.uk.

If you have a side effect not mentioned here that you think may be due to this treatment you can report it to the Medicines Health and Regulatory Authority (MHRA) at yellowcard.mhra.gov.uk.

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Updated: 8 October 2015