Letrozole is a drug used to treat breast cancer in post-menopausal women (women who have gone through the menopause). Taken as a tablet once a day, it’s a type of hormone therapy (also known as endocrine therapy) and belongs to a group of drugs called aromatase inhibitors.

Letrozole is the generic (non-branded) name of the drug. You may also hear it called Femara, which is its most well-known branded name, but there are a number of other brands of letrozole that your doctor may prescribe. All brands contain the same dose of the drug.

Men with breast cancer may also be given letrozole, although tamoxifen is more commonly used.

How does letrozole work?

Some breast cancers are stimulated to grow by the hormone oestrogen. In women who have been through the menopause, oestrogen is no longer produced by the ovaries. But some oestrogen is still made in body fat using an enzyme (a type of protein) called aromatase. Aromatase inhibitors stop this enzyme from working so there’s less oestrogen in the body.

Letrozole will only be prescribed if your breast cancer has receptors within the cell that bind to the hormone oestrogen, known as oestrogen receptor positive or ER+ breast cancer. All breast cancers are tested for oestrogen receptors using tissue from a biopsy or after surgery. When oestrogen binds to these receptors, it can stimulate the cancer to grow.

When oestrogen receptors are not found (oestrogen receptor negative or ER- breast cancer) tests may be done for progesterone (another hormone) receptors. The benefits of hormone therapy are less clear for people whose breast cancer is only progesterone receptor positive (PR+ and ER-). Very few breast cancers fall into this category. However, if this is the case for you your specialist will discuss with you whether letrozole is appropriate.

If your cancer is hormone receptor negative, then letrozole will not be of any benefit.

When is letrozole prescribed?

Letrozole is used to treat post-menopausal women with oestrogen receptor positive primary breast cancer. Taking letrozole helps reduce the chance of breast cancer returning in the same breast or spreading somewhere else in the body. It can also reduce the chances of developing a new breast cancer in the same or opposite breast.

It’s usually given after surgery (known as adjuvant treatment) and following chemotherapy and/or radiotherapy.

Sometimes letrozole is prescribed for primary breast cancer to reduce the size of the cancer before surgery (known as neo-adjuvant endocrine therapy). Before treatment starts, a small metal clip (or marker) may be placed in the area of the breast where the cancer is. This is because in some people the cancer may become difficult to see on a mammogram or ultrasound if it reduces in size, so the marker helps the surgeon find the area again before surgery.

Letrozole may also be prescribed if surgery is not an option for you, for example, if you are not able to have surgery due to other medical conditions. The treatment will not get rid of the breast cancer but can slow its growth and in some people may shrink it. 

It can be used to treat breast cancer that comes back in the chest/breast area (known as local recurrence) or surrounding area (known as locally advanced or regional recurrence). It can also be prescribed if you are diagnosed with secondary breast cancer.

Letrozole is not used on its own as a hormone treatment in pre-menopausal women because it’s not an effective treatment while the ovaries are still making oestrogen. But it’s sometimes given alongside another drug, called goserelin, which stops the ovaries from working.

If there’s any doubt about whether you have gone through the menopause your specialist may recommend a blood test to check this before your hormone therapy is prescribed.

Your specialist team will discuss with you which hormone treatment they recommend for you and why.

How is letrozole taken?

Letrozole is a tablet (2.5mg) that you take once a day. It can be taken with or without food.

It’s best to take it at the same time every day. If you miss a dose, you don’t need to take an extra dose the next day as the level of drug in your body will remain high enough from the previous day. If for any reason you want to stop taking letrozole talk to your specialist first. This is because not taking the drug for the recommended time means you may increase your risk of your breast cancer coming back.

Sometimes it may be possible to change to another hormone tablet.

How long will I have to take letrozole?

In primary breast cancer, if you take letrozole as your only hormone therapy you will usually take it for five years. Some people may be recommended to take letrozole for five years after completing five years of tamoxifen (ten years of hormone therapy in total).

If letrozole is being used to shrink your cancer before surgery, your specialist will advise you how long you can expect to take it for. After taking it for three to four months you will usually have tests to see how the cancer has responded to the letrozole and if it has decreased in size enough for surgery to be performed. After surgery, you will usually continue to take letrozole for a total of five years. If the letrozole has not reduced the size of the cancer, your specialist will talk to you about other treatment options.

The length of time you will be recommended to take letrozole for will vary depending on your circumstances. Ongoing studies are looking at the most effective length of time to take letrozole. You can ask your specialist about this.

If you are taking letrozole to treat breast cancer that can’t be removed by surgery, for local recurrence, locally advanced breast cancer or for secondary breast cancer, you will usually take it for as long as it is keeping your cancer under control. 

Can I take letrozole with other drug?

You should not take other drugs containing oestrogen, such as hormone replacement therapy (HRT), while you are taking letrozole as this may interfere with its effectiveness.

It’s a good idea to talk to your specialist or pharmacist if you are taking any other prescribed or over the counter medicines in case they interact with letrozole. You should also discuss any complementary therapies, herbal remedies or supplements you wish to use before you start them.

Common side effects of letrozole

Everyone reacts differently to drugs and some people have more side effects than others. For most people any side effects usually improve within the first few months of starting the treatment. However, if you have persistent side effects, tell your specialist team so they can help you manage these.

Aching or pain in the joints and/or muscles

One of the most common side effects is aching or pain in the joints and/or muscles.

Symptoms can occasionally be severe but are often mild and temporary, and can usually be relieved by mild pain relief like paracetamol and/or an anti-inflammatory such as ibuprofen. Talk to your doctor before using anti-inflammatory pain relief for this.

Menopausal symptoms

You may have menopausal symptoms such as hot flushes, sweating, mood swings, reduced libido (sex drive) and vaginal dryness. Many people find these symptoms improve over time.

To find out more about coping with these side effects see our information on menopausal symptoms and sex and intimacy.

Low mood/depression

Some people may experience mood changes including feeling low or depressed. It can be difficult to know whether feeling like this is because of the medication or is due to other reasons such as menopausal symptoms or dealing with a diagnosis of cancer.

You can talk to your GP (local doctor) or specialist about how to manage a change in mood.

Find out more about low mood and depression.

Difficulty sleeping

If you have difficulty sleeping (insomnia) you may benefit from simple, practical measures such as limiting caffeine, keeping your room dark and quiet and going to bed and getting up at the same time each day. Relaxation exercises can also be helpful. There are CDs, podcasts and phone apps that can guide you through these techniques. If your insomnia is persistent, your GP may prescribe something to help you sleep.


Fatigue (extreme tiredness) is a commonly reported side effect. Studies show that physical activity can help to relieve fatigue. For more information about the benefits of physical activity during and after breast cancer treatment you can speak to your breast care nurse or call our free Helpline.

Some people report feeling sleepy, although this is less common.

Complementary therapies are helpful for some people.

If you are being treated for secondary breast cancer you can get further information about managing fatigue in our Secondary breast cancer information pack.

Osteoporosis (thinning of the bone)

Letrozole reduces the amount of oestrogen in the body, and lack of oestrogen over time can cause osteoporosis. Your specialist will sometimes check your bone density (strength and thickness) with a scan before or shortly after you start taking letrozole.

Your bone density may also need to be checked around every two years with a repeat scan while you are taking letrozole.

Hypertension (high blood pressure) and high cholesterol

Letrozole can cause hypertension (high blood pressure) in some people. Your GP will be able to monitor and treat this if necessary. It may also cause the level of cholesterol in the blood to rise, although this does not usually need treatment. If you have a history of high cholesterol you may wish to discuss this with your specialist or GP.

Less common side effects of letrozole

Headache, nausea and vomiting

Letrozole can sometimes cause headache, nausea (feeling sick) and vomiting (being sick). While many people find that these problems will improve over time, talk to a member of your specialist team or as they may be able to offer suggestions that will help. Simple pain relief such as paracetamol may help with headache, and anti-sickness medication can be prescribed if you have nausea or vomiting. Taking your letrozole with or after food can help reduce nausea.

Loss of appetite

Some people lose their appetite when taking letrozole. If this happens, it may help to take your tablet with food. You may find eating small frequent meals or snacks helpful in keeping up your food intake. If this is a problem and you struggle to maintain a healthy weight then you can discuss being referred to a dietician with your GP or specialist.

Constipation or diarrhoea

Letrozole can sometimes cause either constipation or diarrhoea. With either problem it’s important to make sure you are drinking enough. Constipation may be helped by keeping active and eating a high-fibre diet. There are also medications that can help with constipation or diarrhoea that your GP or specialist can prescribe.

Carpal tunnel syndrome

Taking letrozole can result in some people developing carpal tunnel syndrome. This is a condition that causes feelings of pain, tingling, coldness or weakness in the hand. Speak to your GP or specialist if you have any of these symptoms.

Hair and skin changes

Some women will notice their hair starts to thin while taking letrozole. However, when you stop taking letrozole your hair will usually return to the way it was before treatment. Some people also report a skin rash.

Vaginal bleeding

Vaginal bleeding can happen in the first few weeks after starting the treatment. It most commonly occurs when someone has changed from one hormone tablet to another. If the bleeding continues for more than a few days, tell your specialist.


Letrozole may cause dizziness. If you feel dizzy you should avoid driving. If this persists see your GP.

You can talk to your GP or specialist if you have any of the side effects mentioned above or other side effects not mentioned, as they may be able offer ways of improving your symptoms. Your breast care nurse may also be able to offer help and support.

Last reviewed: June 2016
Next planned review begins 2018

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