Letrozole

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

Oestrogen can encourage some breast cancers to grow. This is called oestrogen receptor-positive (ER-positive) breast cancer. Letrozole can stop breast cancer cells from growing after menopause.

We explain the most common side effects of this treatment here. We also include some that are less common.

You may get some of the side effects we mention, but you are unlikely to get all of them. And you may have some side effects, including rarer ones, that we have not listed here. 

Other cancer treatments may cause different side effects. If you are also having other cancer treatment, you may have other side effects.

When you have this treatment, you will have regular appointments with a doctor, nurse or pharmacist. Always tell them about any side effects you have. They can give you: 

• drugs to help control most side effects 
• advice about managing side effects. 

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 a different type of hormonal therapy.

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

Your doctor, nurse or pharmacist will give you 24-hour contact numbers for the hospital. If you feel unwell or need advice, you can call at any time of the day or night. Save these numbers in your phone or keep them somewhere safe.

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.

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. 

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

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. 

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.

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 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.

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

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.

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.

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 dizziness. Tell your doctor or nurse if this is a problem.

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.

Some people may find their heartbeat is racing (heart palpitations) or have chest pain when taking letrozole. Speak to your GP or hospital team if you have any of these symptoms. Less commonly, letrozole may cause more serious heart problems. 

Always call 999 if you have:

• chest pain 
• pressure, heaviness, tightness or squeezing across the chest
• difficulty breathing.

If your symptoms are caused by this treatment, you may need steroids. You may be monitored in hospital for a time.

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.

This treatment can make you thirsty and your mouth dry. It can also make the lining of your mouth swollen or sore. Tell your doctor or nurse if this happens. They can suggest things to help.

You may also find that some food tastes different. Your nurse or pharmacist can give you advice.

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.

You may notice changes in your memory. You may also find it harder to concentrate. 

To help you remember things, try using: 

• reminder alarms or notes on your phone 
• a diary, calendar or phone app 
• a notebook you carry with you.

This treatment may affect how your liver works. This is usually mild and goes back to normal after treatment ends. You will have blood tests to check how well your liver is working.

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.