What is letrozole (Femara®)?

Letrozole (Femara®) is a hormonal therapy drug used to treat breast cancer. It is usually used when you have been through a natural menopause. It may also be used when someone is premenopausal and having treatment to stop the ovaries working (temporary menopause). Sometimes it might be used to treat breast cancer in men.

Letrozole may be used:

  • before surgery, to try to reduce the size of the cancer and avoid removal of the breast (mastectomy)
  • after other treatments, to reduce the risk of breast cancer coming back
  • to control breast cancer that has come back or spread to other parts of the body (secondary breast cancer).

It is best to read this information with our general information about hormonal therapies and the type of cancer you have. 

Your doctor will talk to you about this treatment and its possible side effects before you agree (consent) to have treatment. They will explain why you are having letrozole and how long you will take it for.

How letrozole works

Letrozole reduces the amount of a type of hormone called oestrogen in the body. Hormones are substances that our bodies make. They help control how cells and organs work. 

The hormone oestrogen can encourage some breast cancers to grow. This type of breast cancer is called oestrogen receptor-positive (ER-positive) breast cancer. Letrozole can be used in ER-positive breast cancers to stop breast cancer cells from growing when someone has been through the menopause.

Oestrogen is produced mainly in the ovaries before the menopause. After the menopause, the ovaries no longer produce oestrogen. But a small amount of oestrogen is still produced in the fatty tissues, muscle and skin. This happens using a type of protein (an enzyme) called aromatase. Letrozole works by blocking this enzyme to reduce the amount of oestrogen in the body. It is a type of drug called an aromatase inhibitor

If you have not been through the menopause, you may have other types of hormone therapy. 

Taking letrozole tablets

Letrozole comes as tablets you can take at home. You take letrozole once a day, with or without food. Try to take it at the same time each day. 

If you forget to take your tablet, take it as soon as you remember. If it is nearly time for the next dose (within the next 3 hours), just take your usual dose the next day. Do not take a double dose.
You may need to take letrozole for several years. Your nurse or doctor will talk to you about your treatment plan. Always take the tablets exactly as explained. This is important to make sure they work as well as possible for you.

Other things to remember about your tablets:

  • Keep them in the original package and at room temperature, away from heat and direct sunlight.
  • Keep them safe and out of sight and reach of children.
  • If you are sick just after taking the tablets, contact your healthcare team. Do not take another dose.
  • Get a new prescription before you run out of tablets, and make sure you have plenty for holidays.
  • If your treatment is stopped, return any unused tablets to the pharmacist. 

You may have letrozole on its own or with other drugs. Your nurse or doctor will talk to you about your treatment plan. 

Do not stop taking any of your medicines unless your doctor tells you to.

About side effects

We explain the most common side effects of this treatment here. We also include some less common side effects. You may get some of the side effects we mention, but you are unlikely to get all of them. You may have some rarer side effects that we have not listed here.

If you are having other cancer treatments, you may have some side effects that we have not listed here. 

You will see a doctor, nurse or pharmacist regularly while you are having this treatment. Always tell them about any side effects you have. They can give you drugs to help control most side effects. They can also offer advice to help you cope. 

Most side effects can be managed. But sometimes side effects are harder to control. It is important not to stop taking hormonal therapy without telling your doctor. If side effects cannot be managed, your doctor may suggest you take a different type of hormonal therapy.

Serious and life-threatening side effects

Some cancer treatments can cause severe side effects. Rarely, these may be life-threatening. Your cancer doctor or nurse can explain the risk of these side effects to you.

Contact the hospital

Your nurse will give you telephone numbers for the hospital. If you feel unwell or need advice, you can call them at any time of the day or night. Save these numbers in your phone or keep them somewhere safe.

More information

We cannot list every side effect for this treatment. There are some rare side effects that are not listed. You can visit the electronic Medicines Compendium (eMC) for more detailed information.

Common side effects

Hot flushes and sweats

Hot flushes are a common side effect of this treatment. During a flush, your neck and face may feel warm and look red. Flushes may last from a few seconds up to 10 minutes. You may have sweats and then feel cold and clammy. Some people feel anxious or irritable during a hot flush.

There are things you can do to try to reduce flushes:

  • Wear clothes made from natural fabrics, such as cotton.
  • Wear layers of clothes that you can remove if you feel hot.
  • Use cotton bed sheets and have layers of bedding that you can remove if you feel hot.
  • Keep room temperatures cool or use a fan.
  • Have cold drinks rather than hot ones. Try to avoid drinks with caffeine in them.

You may have fewer hot flushes and sweats as your body adjusts to hormonal treatment. Or your doctor can prescribe drugs to help. Flushes and sweats usually stop a few months after treatment finishes, but some people continue to have them.

We also have information about coping with hot flushes caused by the menopause in our information about managing menopausal symptoms.

Feeling tired (fatigue)

Feeling tired is a common side effect. Try to pace yourself and plan your day so you have time to rest between activities.

Being physically active can help to manage tiredness and give you more energy. It also:

  • helps you sleep better
  • reduces stress
  • improves your bone health.

If you feel sleepy, do not drive or operate machinery.

Muscle or joint pain

You may get pain in your muscles or joints. If this happens, tell your doctor so they can give you painkillers. Being physically active may help with the pain and keep your joints flexible. Keeping to a healthy weight may help too. Tell your doctor or nurse if the pain does not get better.

Feeling sick, and indigestion or tummy pain

Any sickness is usually mild, but let your doctor or nurse know if this happens. Letrozole may also cause indigestion or tummy pain. Tell your doctor or nurse if you have these symptoms. They can prescribe drugs to help.

Raised cholesterol level

This treatment can raise the level of cholesterol (a fatty substance) in the blood. You may have a blood test to check the level of cholesterol. Your doctor may give you medicines to control your cholesterol levels.

Change in appetite

You may find your appetite increases when you are taking letrozole. Eating well and keeping active will help if you have concerns about your weight. If you do not have much appetite, try eating small meals often. If problems with eating do not get better, talk to your doctor or nurse.

Hair thinning

Your hair may become thinner when you are taking this treatment. This is usually mild. Ask your nurse for advice if you are worried about this.

Mood changes

You may have some mood changes during this treatment. You may feel low or depressed. Let your doctor or nurse know if you notice any changes.

Skin changes

This treatment may affect your skin. It may cause a rash, which might be itchy. Your doctor or nurse can tell you what to expect. If your skin feels dry, try using an unperfumed moisturising cream every day.

Always tell your doctor or nurse about any changes to your skin. They can give you advice and may prescribe creams or medicines to help. Skin changes usually improve when treatment finishes.

Headaches

This treatment may cause headaches. If you have headaches, tell your doctor. They may give you painkillers to help.

Feeling dizzy

This treatment may cause dizziness. Tell your doctor or nurse if this is a problem.

Weight gain

You may gain weight when you are having this treatment. Eating healthily and being active can help you keep to a healthy weight. Your doctor, nurse or a dietitian can give you more advice.

Vaginal bleeding or dryness

This treatment can cause vaginal changes. You may have bleeding when you start this treatment, or if you change to it from another treatment. If bleeding continues for more than a few days, tell your doctor or nurse.

You can use non-hormonal creams, gels or lubricants to help with vaginal dryness and any discomfort during sex. You can buy these from a chemist, or your doctor can prescribe them. We have more information about cancer and sex.

Diarrhoea

This treatment may cause diarrhoea. Diarrhoea means passing more stools (poo) than is usual for you, or having watery or loose stools. If you have a stoma, it will be more active than usual.

If you have diarrhoea:

  • try to drink at least 2 litres (3½ pints) of fluids each day
  • avoid alcohol, caffeine, milk products, high-fat foods and high-fibre foods
  • contact the hospital for advice.

Constipation

This treatment can cause constipation. Constipation means that you are not able to pass stools (poo) as often as you normally do. It can become difficult or painful. Here are some tips that may help:

  • Drink at least 2 litres (3½ pints) of fluids each day.
  • Eat high-fibre foods, such as fruit, vegetables and wholemeal bread.
  • Do regular gentle exercise, like going for short walks.

If you have constipation, contact the hospital on the 24-hour number for advice. They can give you drugs called laxatives to help. 

If you have not been able to pass stools for over 2 days and are being sick, contact the 24-hour number straight away. 

Bone thinning (osteoporosis)

If you take this treatment for several months or more, you may get bone thinning. This is called osteoporosis. This can increase your risk of a broken bone (fracture). You may have bone density scans to check your bone health before and during treatment.

Doing regular exercise, such as walking, can improve your bone health. Eating a healthy diet can help too. Your doctor may prescribe drugs called bisphosphonates to help protect your bones. They may also advise you to take calcium and vitamin D supplements.

Raised blood pressure

Tell your doctor or nurse if you have ever had any problems with your blood pressure. Your nurse will check it regularly during your treatment.

Build-up of fluid

You may get swollen feet and ankles due to fluid building up. If you notice this or any other swelling, tell your doctor.

Less common side effects of letrozole

Effects on the heart

This treatment can affect the way your heart works. Your doctor may do tests to see how well your heart is working. You may have these tests before, during, and sometimes after treatment.

Contact a doctor straight away if you:

  • have pain or tightness in your chest
  • feel breathless or dizzy
  • feel your heart is beating too fast or too slowly.

Other conditions can cause these symptoms. But it is important to get them checked by a doctor.

Risk of infection

Letrozole can reduce the number of white blood cells in your blood. These cells fight infection. If the number of white blood cells is low, you are more likely to get an infection. 

If you have an infection, it is important that it is treated as soon as possible. Contact your GP if you think you have possible symptoms of infection. Signs of an infection can include:

  • having a temperature over 37.5°C (99.5°F) 
  • feeling shaky
  • a cough
  • a sore throat
  • needing to pass urine (pee) often. 

You may have blood tests during treatment.

Eye problems

This treatment can cause your eyes to feel irritated. It can also cause blurry vision. Tell your doctor if you notice any problems with your eyes or eyesight.

Difficulty sleeping

This treatment can affect your sleep. Tell your doctor if you are having problems. If you are feeling sleepy, do not drive or operate machinery.

Memory and concentration

You may notice changes in your memory. You may also find it harder to concentrate. To help you remember things, try using:

  • a notebook
  • notes on your phone
  • a calendar.

Effects on the liver

This treatment may affect how your liver works. This is usually mild. You will have blood tests to check how well your liver is working.

Dry mouth

This treatment can make you feel thirsty and your mouth feel dry. Tell your doctor or nurse if your mouth becomes dry or sore. They can suggest things to help.

Changes to your taste

You may get a bitter or metal taste in your mouth. Sucking sugar-free sweets may help with this. Some foods may taste different or have no taste. Try different foods to find out what tastes best to you. Taste changes usually get better after treatment finishes. Your nurse can give you more advice.

Other information

Blood clot risk

Cancer and some cancer treatments can increase the risk of a blood clot. Contact the hospital straight away on the 24-hour number if you have any of these symptoms during or after treatment:

  • throbbing pain or swelling in a leg or arm
  • reddening of the skin in the area – if you have black or brown skin, this can be harder to notice, but the skin might become darker
  • suddenly feeling breathless or coughing.

Always call 999 if you have:

  • chest pain
  • difficulty breathing.

A blood clot is serious, but it can be treated with drugs called anticoagulants. These thin the blood. Your doctor, nurse or pharmacist can give you more information about preventing and treating blood clots.

Lactose

Letrozole contains a sugar found in cow’s milk (lactose). Tell your doctor if you have an allergy to lactose.

Contraception

Letrozole is usually used when you have been through a natural menopause. However, if you have recently been through the menopause or are having treatment to stop your ovaries working (temporary menopause), there is still a very small chance you may become pregnant. You doctor can tell you more about this.

If you are sexually active, it is important not to get pregnant while having this treatment. The drug may harm the developing baby. 

Even if you have had your menopause, your doctor will discuss using effective contraception during your treatment.

Breastfeeding

You are advised not to breastfeed while having this treatment, or for some time after treatment finishes. This is because the drugs could be passed to the baby through breast milk.

Your doctor or nurse can give you more information.

Medical and dental treatment

If you need medical treatment for any reason other than cancer, always tell the doctors and nurses you are having cancer treatment. Give them the contact details for your cancer doctor so they can ask for advice.

If you think you need dental treatment, talk to your cancer doctor or nurse. Always tell your dentist you are having cancer treatment.

Other medicines

Some medicines can affect how this treatment works or be harmful while you are having it. Always tell your cancer doctor, nurse or pharmacist about any drugs you are taking or planning to take, such as: 

About our information

  • Reviewers

    This information has been written, revised and edited by Macmillan Cancer Support’s Cancer Information Development team. It has been reviewed by expert health professionals and people living with cancer.

    Our cancer information has been awarded the PIF TICK. Created by the Patient Information Forum, this quality mark shows we meet PIF’s 10 criteria for trustworthy health information.

Date reviewed

Reviewed: 01 November 2022
|
Next review: 01 May 2025
Trusted Information Creator - Patient Information Forum
Trusted Information Creator - Patient Information Forum

Our cancer information meets the PIF TICK quality mark.

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