Letrozole (Femara®)

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. 

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.

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 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.

You may get pain in your muscles or joints. If this happens, tell your doctor or nurse. They can give you advice and painkillers or other drugs to help. Being physically active and keeping to a healthy weight may help with the pain. It can keep your joints flexible and reduce stress on them. If the pain does not get better, tell your doctor or nurse. They may talk to you about changing to a different hormonal drug.

This treatment can raise the level of cholesterol. This is a fatty substance in the blood. You may have a blood test to check the level of cholesterol. Your doctor may refer you to a dietitian for advice on diet, or prescribe medicines to control your cholesterol levels.

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.

While you are having this treatment, your hair may become thinner, drier and more brittle. This is usually mild. If you are worried about this, ask your nurse for advice on hair care.

You may feel low or depressed, or have mood swings, during this treatment. Talking to family and friends about how you feel might help. If mood changes last for more than a few weeks, tell your doctor, nurse or pharmacist. They can talk to you about different ways to manage low mood or depression.

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

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

This treatment may cause headaches. If you have headaches, tell your doctor, nurse or pharmacist. They can give you advice about painkillers that may help. Tell them if the headache does not get better, or gets worse.

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

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 dietitian can give you more advice.

This treatment can cause vaginal dryness. Non-hormonal creams, gels or lubricants can help with vaginal dryness and any discomfort during sex. You can buy these from a chemist, or your doctor can prescribe them.

You might have slight vaginal bleeding for a short while:

• when you start treatment
• if you change from 1 hormonal treatment to another.

Vaginal dryness can also cause bleeding. Tell your doctor or nurse if you have any vaginal bleeding, especially if it lasts for more than a few days.

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

If you are passing loose stools 3 or more times a day and this is not normal for you, contact the hospital as soon as possible on the 24-hour number. Follow the advice they give you about:

• taking anti-diarrhoea medicines 
• drinking enough fluids to keep you hydrated and to replace lost salts and minerals
• any changes to your diet that might help. 

They might also ask you for a specimen of your stool to check for infection.

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. 

Taking this treatment for several months or more can increase the risk of bone thinning. This is called osteoporosis. It can make you more likely to get a broken bone (fracture). 

You may have bone density scans to check your bone health before and during treatment. These scans can diagnose osteoporosis or osteopenia (low bone density) so that it can be monitored. If your scan shows osteoporosis, your doctor usually prescribes:

• drugs to protect your bones called bisphosphonates
• calcium and vitamin D supplements. 

Regular exercise where you support your body weight, like walking, running and dancing, can help look after your bones. This is called weight-bearing exercise. Other types of strength exercises are also helpful. If you have any bone thinning or the cancer is affecting your bones, get advice from your cancer team first. 

Eating a healthy diet and not smoking also helps take care of your bones.

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.

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

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.

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.

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

Your doctor, nurse or pharmacist can give you more information.

If you need medical treatment for any reason other than cancer, always tell the healthcare professional that you are having cancer treatment. Give them the contact details for your cancer doctor or cancer team so they can ask for advice.

If you have appointments with a dentist, always tell them you are having cancer treatment. Talk to your cancer team before you have any dental treatment.

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: 

• medicines you have been prescribed
• medicines you buy in a shop, pharmacy or online
• vitamins or supplements 
• herbal drugs and complementary or homeopathic therapies
• recreational drugs – for example, cannabis.