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Letrozole (Femara)

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This page tells you about the hormone therapy letrozole and its possible side effects. You can find information about


What letrozole is

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

Many breast cancers are stimulated to grow by the female sex hormones oestrogen and progesterone. These breast cancers are called hormone sensitive or hormone receptor positive and can be treated with drugs that block the effects of these hormones.

Women who have had their menopause don’t produce oestrogen from their ovaries. But they still produce a small amount by turning other sex hormones called androgens into oestrogen. Androgens are made by your adrenal glands, the small glands above your kidneys. Androgens need an enzyme called aromatase to turn them into oestrogen. This change happens mainly in fatty tissue, muscle and the skin. Aromatase inhibitors stop (inhibit) aromatase, so it can’t change the androgen into oestrogen. These drugs are only suitable for women who have had their menopause.


How you have letrozole

You take letrozole as a tablet, once a day. It is best to take them 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. Do not take a double dose.

You may find it helpful to read our general information about hormone therapies.

We've listed the side effects associated with letrozole below. Most of them are mild to moderate and they generally disappear after a few days to a few weeks of treatment.

You can use the links to find out more about each side effect. Where there is no link, please go to the cancer drug side effects section or click on search at the top of the page.


Common side effects

More than 10 in every 100 women have one or more of these.

  • Hot flushes and sweats – this happens in about 3 out of 10 women (30%)
  • Pain in joints or bones – this affects about 1 out of 5 women (20%)
  • Tiredness and weakness (fatigue) – about 1 in 5 women (20%) have this side effect
  • Increased levels of cholesterol in the blood – this is usually only slightly increased. You will have regular blood tests to check the levels

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 – in about 1 in 10 women (10%)
  • Dizziness occurs in around 1 in 10 women (10%)
  • Generally feeling unwell (malaise)
  • Feeling or being sick – this is usually mild and can be easily controlled by anti sickness tablets, or if necessary injections. It affects about 1 in 10 women (10%)
  • Fluid retention causing ankle and or finger swelling – this affects about 1 in 10 women (10%)
  • Loss of appetite or indigestion
  • Hair thinning
  • Diarrhoea – if this happens it is usually mild. You should drink plenty of fluids. If it gets severe or persistent you could become dehydrated, so you should tell your doctor or nurse
  • Constipation occurs in some people
  • Vaginal dryness – let your doctor or nurse know if you have this as medicines can help
  • Lower interest in sex (reduced libido)
  • Sadness or depression
  • A cough and breathlessness – this affects less than 1 in 10 (10%) of women 
  • 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 have changed 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 1 in 20 (5%) women treated
  • 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, 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 in one or both hands, particularly in the fingers and thumb. This condition is called carpal tunnel syndrome and 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

The side effects above 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)
  • Other drugs you are having

Coping with side effects

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.


Letrozole is for women who have had their menopause. But if there is any possibility that you could become pregnant you must use reliable contraception while taking this treatment. Letrozole could harm a developing baby. Talk to your doctor or nurse about contraception before starting the treatment.


Do not breastfeed during this treatment because the drug may come through in the breast milk.

Lactose intolerance

Letrozole tablets contain lactose (milk sugar). If you have an intolerance to lactose, contact your doctor before taking this medicine.


More information on letrozole

This page does not 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 www.mhra.gov.uk.

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Updated: 20 September 2013