Hormones are substances produced naturally in the body. They act as chemical messengers and help control the activity of cells and organs. Hormonal therapies are drugs that interfere with the way hormones are made or how they work in the body.

Many breast cancers rely on the hormone oestrogen to grow. This type of breast cancer is called oestrogen receptor-positive (ER positive) breast cancer.

In women, after the menopause, oestrogen is no longer made in the ovaries. Instead, it is made in the fatty tissues of the body. This happens when an enzyme (a type of protein) called aromatase changes other hormones into oestrogen. Letrozole is a type of drug called an aromatase inhibitor. It blocks (inhibits) the activity of aromatase and reduces the amount of oestrogen in the body.

Letrozole is used to treat oestrogen-receptor positive (ER positive) breast cancer in women who have been through the menopause. It is sometimes used to treat breast cancer in men.

You may take letrozole after surgery and other treatments to reduce the risk of breast cancer coming back. You will usually take it for a few years. Doctors sometimes prescribe it before or after you have another type of hormonal therapy drug.

Sometimes doctors give letrozole before surgery to try to reduce the size of the cancer and avoid having a mastectomy (removal of the breast). Letrozole is also used to control breast cancer that has spread to other parts of the body (secondary breast cancer).

You take letrozole as a tablet once a day. You can take it with or without food. Try to take it at the same time each day, morning or evening. Always take your tablets exactly as your nurse or pharmacist explained. This is important to make sure they work as well as possible for you.

Do not stop taking any of your tablets unless your doctor tells you to. Here are some important things to remember:

• If you forget to take your tablets, just take your usual dose the next day. Don’t take a double dose.
• Keep tablets in the original package and at room temperature, away from heat and direct sunlight.
• Keep them safe and out of the sight and reach of children.
• Get a new prescription before you run out of tablets and make sure you have plenty for holidays.
• Return any remaining tablets to the pharmacist if your treatment is stopped.

We explain the most common side effects of letrozole here. We also include some rarer side effects. You may get some of the side effects we mention, but you are very unlikely to get all of them. If you are having other drugs as well, you may have some side effects that we don’t list here.

You will see a doctor or nurse regularly while you have this treatment. Always tell your doctor or nurse about the side effects you have. They can prescribe drugs to help control them and give you advice about managing them. Don’t stop taking letrozole without talking to your doctor first.

More information about this drug

We’re not able to list every side effect for this treatment here, particularly the rarer ones. For more detailed information you can visit the electronic Medicines Compendium.

Hot flushes and sweats

These are common and are often mild, but this can vary. Hot flushes and sweats may lessen after the first few months. Cutting down on nicotine, alcohol and hot drinks containing caffeine, such as tea and coffee, can help. Dress in layers, so you can remove clothes as needed. Natural fabrics, such as cotton, may feel more comfortable.

If hot flushes are a problem, tell your doctor. Low doses of certain antidepressant drugs can help to reduce flushes.

Women can read more about coping with hot flushes in our section on managing menopausal symptoms.

Men can read more about coping with hot flushes in our section on managing hormonal symptoms.

Bone thinning

Taking letrozole for a few years increases your risk of bone thinning, called osteoporosis. This can increase your risk of a broken bone (fracture). You will usually have a bone density scan to check your bone health before and during treatment. If you are at risk of osteoporosis, your doctor may prescribe drugs called bisphosphonates to protect your bones. They will also usually advise you to take calcium and vitamin D supplements. Regular walking, eating a healthy diet, not smoking and sticking to sensible drinking guidelines will also help to strengthen your bones.

Tiredness and lack of energy

You may feel tired, sleepy or feel like you have no energy when you start taking letrozole. Try to pace yourself until this improves. It’s important to get the right balance of having enough rest and being physically active. Regular short walks will help you to feel less tired. If you feel sleepy, don’t drive or operate machinery.

Joint and muscle pain

You may have pain and stiffness in your joints, and sometimes in your muscles, while taking letrozole. Let your doctor or nurse know if this happens. They can prescribe painkillers and give you advice. Being physically active and maintaining a healthy weight can help reduce joint pain and keep them flexible. Let your doctor know if it doesn’t 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. Let them know if you have any of these symptoms. They can prescribe drugs to help.

Change in appetite

If you don’t have much appetite, try eating small meals often and regularly. If problems with eating don’t get better talk to your doctor or nurse.

Some women find their appetite increases. Eating healthily and being physically active will help if you have concerns about your weight.

Hair thinning

You may notice that your hair becomes thinner while taking letrozole. This is usually mild. Your hair will get thicker after treatment finishes.

Skin rashes

You may get a mild skin rash. Tell your doctor or nurse if this happens. It’s very important to contact your doctor straightaway if you get a severe skin rash.

Mood and behaviour changes

Some people find they have a low mood while taking letrozole. Or you may have problems concentrating, feel anxious or have difficulty sleeping. Talk to your nurse or doctor if you have these changes so they can give you support and advice.

Headaches

If you have headaches let your doctor or nurse know. They can usually be controlled with painkillers you can buy yourself.

Feeling dizzy

Letrozole may cause dizziness. Let your doctor or nurse know if this is a problem.

Weight gain

You may put on weight when you’re taking hormonal therapy. Eating healthily and being more physically active can help to keep you to a healthy weight. Your nurse can give you more advice.

Vaginal bleeding

For some women letrozole causes vaginal bleeding. If this happens it is most likely in the first few weeks of treatment, or when you change from another hormonal therapy to letrozole. If bleeding continues for more than a few days, tell your doctor or nurse.

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.

Raised cholesterol level

Your doctor may check your cholesterol levels with a blood test.

Build-up of fluid

You may get swollen feet and ankles because of fluid building up. If you notice this or any other swelling let your doctor know.

Vaginal dryness

For some women, letrozole causes vaginal dryness. Non-hormonal creams and gels or lubricants can help reduce dryness and discomfort during sex. You can buy these at a chemist or your doctor can prescribe them.

Urine infection

Let your doctor know if you have pain or discomfort when you pass urine, if you need to go more often, or your urine is cloudy or smelly. Drink lots of fluids if you think you may have an infection.

Eye problems

Some people get sore eyes or blurry vision with letrozole. Always tell your doctor or nurse if you notice any change in your vision.

Risk of heart problems

If you notice changes to your heartbeat, such as it speeding up, let your doctor know. If you have pain or tightness in your chest, or feel breathless at any time during or after treatment, see a doctor straightaway.

Always let your doctor or nurse know about any side effects you have. There are usually ways in which they can be controlled or improved.

If you have ongoing problems

Usually side effects settle within a few months of starting letrozole, or they can be controlled or managed. But if side effects continue, make sure you talk to your cancer doctor or nurse. They can usually suggest ways to improve your side effects. If things don’t improve, get back to your doctor or nurse. They may suggest you try something else.

It’s really important not to stop taking letrozole without telling your doctor, as this may affect the success of your treatment. If your side effects can’t be managed, your cancer doctor may suggest you take a different type of hormonal therapy.

Other medicines

Letrozole can interact with other drugs. This includes medicines you can buy in a shop or chemist. Tell your doctor about any medicines you are taking. These include ones you can buy yourself, complementary therapies and herbal drugs.

Medical or dental treatment

If you need to go into hospital for any reason other than cancer, always tell the doctors and nurses that you are taking letrozole. Explain you are taking hormonal therapy that no one should stop or restart without checking with your cancer doctor first. Tell them the name of your cancer doctor so they can ask for advice.

Always tell your dentist you are taking letrozole before having any dental treatment.