What is anastrozole?

Anastrozole 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.

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

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

How does anastrozole 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.

Anastrozole 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 anastrozole is appropriate.

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

When is anastrozole prescribed? 

Anastrozole is used to treat post-menopausal women with oestrogen receptor positive primary breast cancer. Taking anastrozole 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.

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.

Anastrozole 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 anastrozole taken?

Anastrozole is a tablet (1mg) 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 anastrozole it’s important that you 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 anastrozole? 

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

It can also be taken for two to three years following two to three years of tamoxifen (five years of hormone therapy in total).

The length of time you will be recommended to take anastrozole for will vary depending on your circumstances. You can ask your specialist about this.

If you’re taking anastrozole to treat local recurrencelocally advanced breast cancer or secondary breast cancer, you will usually take it for as long as it’s keeping the cancer under control. 

Can I take anastrozole with other drugs? 

You should not take other drugs containing oestrogen, such as hormone replacement therapy (HRT), while you are taking anastrozole 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 anastrozole. You should also discuss any complementary therapies, herbal remedies or supplements you wish to use before you start them.

Common side effects of anastrozole

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 also 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 may want to see our tips on secondary breast cancer and managing fatigue can get further information in our Secondary breast cancer resource pack.

Osteoporosis (thinning of the bone)

Anastrozole 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 anastrozole.

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

Less common side effects of anastrozole

Headache, nausea and vomiting

Anastrozole can sometimes cause headache, nausea (feeling sick) and vomiting (being sick). While many people find that these problems improve over time, talk to a member of your specialist team or GP 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 anastrozole with or after food can help reduce nausea.

High cholesterol

Anastrozole may 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.

Loss of appetite

Some people lose their appetite when taking anastrozole. 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 you can discuss being referred to a dietician with your GP or specialist.

Constipation or diarrhoea

Anastrozole can sometimes cause either constipation or diarrhoea. With either problem it’s important to make sure you’re 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 anastrozole can result in some people developing carpal tunnel syndrome. This is a condition that causes 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 anastrozole. However, when you stop taking anastrozole 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.


Anastrozole 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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