Anastrozole (Arimidex®)

Anastrozole is a hormonal therapy drug used to treat breast cancer. It’s best to read this with our general information about breast cancer.

You usually have anastrozole as tablets and as an outpatient. Your cancer doctor or nurse will tell you how long you take it for.

Like all hormonal therapy drugs, anastrozole can cause side effects. Some of these can be serious so it’s important to read the detailed information below. How hormonal therapy affects people varies from person to person. Your doctor or nurse can talk to you more about this and give you advice on how to manage side effects.

Tell your doctor or nurse straight away if you feel unwell or have severe side effects, including any we don’t mention here. If you need to see a health professional for any reason other than cancer, always tell them that you are having this treatment.

How anastrozole works

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.

After a woman goes through 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. Anastrozole 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.

When anastrozole is given

Anastrozole is used to treat breast cancer in women who have been through the menopause. Sometimes it is used to treat breast cancer in men.

Anastrozole may be used after other treatments to reduce the risk of breast cancer coming back.

It is also used to control breast cancer that has come back or spread to other parts of the body (known as secondary breast cancer).

Your doctor or nurse will explain why you are having anastrozole and how long you will take it for.

Taking your anastrozole tablets

You take anastrozole as a tablet once a day, with or without food. You 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.

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 the tablets in their original packaging 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.

Possible side effects of anastrozole

We explain the most common side effects of anastrozole 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 any side effects you may have. They can prescribe drugs to help control them and give you advice about managing them. Don’t stop taking anastrozole without talking to your doctor first.

More information about this drug

We are 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 (eMC).

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.

There are things you can do to try to reduce flushes:

  • Try to cut down on nicotine, alcohol and hot drinks containing caffeine, such as tea and coffee.
  • Dress in layers, so you can remove clothes as needed.
  • Wear clothes made from natural fabrics, such as cotton.

If hot flushes are troubling you, tell your doctor or nurse. Low doses of certain anti-depressant 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.

Joint and muscle pain

You may have pain and stiffness in your joints, and sometimes in your muscles, while taking anastrozole. Tell your doctor or nurse if this happens. They can prescribe painkillers and give you advice.

Being physically active and keeping to a healthy weight can help reduce joint pain and keep them flexible. Let your doctor or nurse know if it doesn’t get better.

Tiredness and lack of energy

You may feel tired, sleepy or feel like you have no energy when you start taking anastrozole. Try to pace yourself until this improves. Aim for a balance between having enough rest and being physically active. Doing exercise, like going for regular short walks, will help you feel less tired. 

It is important not to drive or use machinery if you feel sleepy.

Feeling sick (nausea) or being sick (vomiting)

Any sickness is usually mild, but let your doctor or nurse know if this happens. Try taking your anastrozole tablets with food or at night. 

Loss of appetite 

If you don’t have an appetite or can’t face big meals, try to eat small, frequent meals or snacks instead of three meals a day.

Taste changes

You may get a bitter or metallic taste in your mouth or find that food tastes different. This should go away when your treatment finishes. Your doctor or nurse can give you more advice. 


This is usually mild. If it doesn’t get better, your doctor can prescribe drugs to control it. Make sure you drink at least two litres (about three and a half pints) of fluids every day if you have diarrhoea.


If you have headaches, let your doctor or nurse know. They can usually be controlled with painkillers you can buy from a chemist.

Skin rashes

You may get a mild skin rash while taking anastrozole. Tell your doctor or nurse if this happens. 

It is very important to contact your doctor or nurse straight away if you get a severe rash.

Hair thinning

Your hair may become thinner while taking anastrozole. Your hair will get thicker after treatment finishes.

Vaginal bleeding or dryness

Anastrozole can cause vaginal bleeding or dryness. If bleeding happens, it is usually in the first few weeks of treatment or when you change from another hormonal therapy to anastrozole. If it continues for more than a few days, tell your doctor or nurse. 

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

Pain, numbness and tingling in hand and fingers

These symptoms may be due to carpal tunnel syndrome, which is caused by pressure on a nerve in the wrist. It is more common in people taking anastrozole. Tell your doctor if you have these symptoms.

Raised cholesterol level

Anastrozole can raise the level of cholesterol (a fatty substance) in your blood. Your doctor may check this with a blood test.

Liver changes

Anastrozole can sometimes cause changes in the way your liver works. Your doctor will arrange for you to have regular blood tests to check your liver is working properly. You are very unlikely to notice any problems. 

Possible side effects of long-term anastrozole treatment

Bone thinning (osteoporosis)

Taking anastrozole for a few years increases your risk of bone thinning. This is called osteoporosis. This can increase your risk of a broken bone (fracture). You will usually have bone density scans to check your bone health before and during treatment.

If you are at risk of osteoporosis, your doctor may prescribe drugs called bisphosphonates to help protect your bones. They will also usually advise you to take calcium and vitamin D supplements.

You can also help strengthen your bones by:

Side effects can usually be controlled or improved. Always let your doctor or nurse know about any side effects you have so they can help you feel better.

Other information about anastrozole

If you have ongoing side effects

Most of the time side effects can be controlled or managed. But for a few people, they can be more troubling. If this happens, make sure you talk to your doctor or nurse. They can usually suggest ways to improve your side effects. If things don’t improve, go back to your doctor or nurse. They may suggest you try something else.

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

Other medicines

Anastrozole 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, vitamins and herbal drugs.

Problems with lactose

Anastrozole tablets contain a type of sugar called lactose. If you have been told by a doctor that you cannot digest lactose (lactose intolerant) or other sugars, talk to your doctor before taking this drug.

Medical or dental treatment

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

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