Letrozole (Femara)

Letrozole is a type of hormone therapy drug. It is also known as Femara. 

It is a treatment for breast cancer.

Letrozole is pronounced let-roe-zole.

How does letrozole work?

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.

Letrozole only works in women who have had their menopause.

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

How do you have letrozole?

You have letrozole as a tablet. 

You must take tablets according to the instructions your doctor or pharmacist gives you.

You should take the right dose, not more or less.

Talk to your healthcare team before you stop taking a cancer drug, or if you miss a dose.

How often do you have letrozole?

Usually, you take letrozole once a day.

You take letrozole for 5 years or until the cancer comes back. If you have advanced breast cancer then you take letrozole until the cancer starts to grow again.


You might have blood tests before starting treatment and during your treatment. They check your general health and might check your levels of blood cells and other substances in the blood.

What are the side effects of letrozole?

Side effects can vary from person to person. They also depend on what other treatment you are having. 

When to contact your team

Your doctor, nurse, or pharmacist will go through the possible side effects. They will monitor you closely during treatment and check how you are at your appointments. Contact your advice line as soon as possible if:

  • you have severe side effects 

  • your side effects aren’t getting any better

  • your side effects are getting worse

Early treatment can help manage side effects better.

We haven't listed all the side effects here. Remember it is very unlikely that you will have all of these side effects, but you might have some of them at the same time.

Common side effects

These side effects happen in more than 10 in 100 people (more than 10%). You might have one or more of them. They include:

Raised cholesterol levels 

You can have raised cholesterol levels in the blood. This is usually only slightly raised and you will have regular blood tests to check the levels.

Hot flushes

We have some tips for coping with hot flushes in women. We also explain some of the possible treatments. Talk to your doctor if your hot flushes are hard to cope with. They might be able to prescribe you some medicines.

Increased sweating

You might sweat more than usual especially under the arms and your hands and feet. Tell your doctor or nurse they may be able to help.  

Joint pain

You might feel some pain from your joints. Speak to your doctor or nurse about what painkillers you can take to help with this.

Tiredness (fatigue)

You might feel very tired and as though you lack energy.

Various things can help you to reduce tiredness and cope with it, for example exercise. Some research has shown that taking gentle exercise can give you more energy. It is important to balance exercise with resting.

Occasional side effects

These side effects happen in between 1 and 10 out of every 100 people (between 1 and 10%). You might have one or more of them. They include:

  • change in appetite - you might eat more or less
  • depression
  • headaches
  • dizziness
  • feeling like your heart is racing (heart palpitations)
  • high blood pressure
  • feeling or being sick
  • indigestion
  • constipation or diarrhoea
  • tummy (abdominal) pain
  • hair loss
  • rash
  • dry skin
  • bone problems such as weak bones, pain or fractures
  • arthritis
  • bleeding from the vagina
  • swelling of hands, feet, arms and legs
  • chest pain
  • weight gain
  • muscle pain

Rare side effects

This side effects happens in fewer than 1 in 100 people (fewer than 1%). You might have one or more of them. They include:

  • urine infection
  • pain where the cancer is
  • a drop in white blood cells
  • anxiety, feeling nervous and irritable
  • feeling sleepy
  • not being able to get to sleep
  • not being able to remember
  • damage to the nerves causing tingling, prickling or numbness
  • loss of feeling of touch or sensation of touch
  • loss of taste
  • stroke
  • eye problems such as blurred vision, cataracts or irritation
  • heart problems such as a fast heart rate, angina or heart attack
  • blood clots including deep vein clots (DVT)
  • shortness of breath and a cough
  • dry mouth and inflammation of the lining of the mouth
  • changes to the way the liver works
  • needing to pass small amounts of urine frequently during the day
  • discharge from the vagina
  • dryness of the vagina
  • breast pain
  • general swelling
  • feeling thirsty
  • high temperature
  • weight loss
  • itchy skin
  • raised, itchy rash

Coping with side effects

We have more information about side effects and tips on how to cope with them.

What else do I need to know?

Other medications, food and drink

Cancer drugs can interact with medicines, herbal products, and some food and drinks. We are unable to list all the possible interactions that may happen. An example is grapefruit or grapefruit juice which can increase the side effects of certain drugs.

Tell your healthcare team about any medicines you are taking. This includes vitamins, herbal supplements and over the counter remedies. Also let them know about any other medical conditions or allergies you may have.


You should only have this treatment if you have gone through the menopause. Because of the way it works there is a very small chance you could become pregnant. Your doctor will talk to you about the most effective contraception to use. 

Pregnancy and contraception

You should only take this drug if you have been through your menopause. When you first go through the menopause there can still be a chance that you could become pregnant. This drug may harm a baby developing in the womb. It is important to not to become pregnant while you are having treatment and for a few months afterwards. So, even if you have had your menopause, talk to your doctor or nurse about effective contraception before starting treatment. 


Don’t breastfeed during this treatment because the drug may come through into your breast milk.

Treatment for other conditions

If you are having tests or treatment for anything else, always mention your cancer treatment. For example, if you are visiting your dentist.

More information about this treatment

For further information about this treatment and possible side effects go to the electronic Medicines Compendium (eMC) website. You can find the patient information leaflet on this website.

You can report any side effect you have to the Medicines Health and Regulatory Authority (MHRA) as part of their Yellow Card Scheme.

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