Anastrozole (Arimidex)

Anastrozole is also called Arimidex. It is a type of hormone therapy called an aromatase inhibitor and treats breast cancer in women who have had their menopause.

Many breast cancers are stimulated to grow by the female sex hormones oestrogen and progesterone. These breast cancers can be treated with drugs that block the effects of these hormones.

In women who have had their menopause, oestrogen is mainly produced by changing androgens (sex hormones produced by the adrenal glands) into oestrogens. This process is called aromatisation and happens mainly in the fatty tissues, muscle and the skin. It needs a particular enzyme called aromatase. 

Arimidex blocks the process of aromatisation. So it lowers the amount of oestrogen in the body. In advanced breast cancer the cancer cells may grow more slowly or stop growing completely.

You take anastrozole as a tablet, once a day.  

It is very important that you take tablets according to the instructions your doctor or pharmacist gives you. For example, whether you have a full or empty stomach can affect how much of a drug gets into your bloodstream. You should take the right dose, not more or less. And never stop taking a cancer drug without talking to your specialist first.

We've listed the side effects associated with anastrozole below. You can use the links to find out more about each side effect. Where there is no link, please use the search box at the top of the page. Or look at the cancer drug side effects section.

More than 10 in every 100 people have one or more of these.

• Painful joints happen in more than 3 out of 10 women (30%)
• Mood changes happen in up to 2 in 10 women (20%)
• Tiredness or fatigue happens in up to 2 out of 10 women (20%) while they are taking anastrozole
• Menopausal symptoms, including hot flushes and sweats, occur in more than 1 out of 10 women (10%)
• A lowered interest in sex (reduced libido) occurs in more than 1 out of 10 women (10%)
• Skin rashes occur in more than 1 out of 10 women (10%) – these are usually mild but let your doctor or nurse know if you have a rash
• Feeling or being sick happens in about 1 in 10 women (10%) – it is usually mild and can usually be controlled by anti sickness medicines
• Headaches
• Bone pain
• Painful hands with a weakened grip and numbness and tingling – carpal tunnel syndrome
• Hair thinning may happen, but is usually mild
• Loss of appetite

Between 1 and 10 in every 100 people have one or more of these.

• A cough and breathlessness affects less than 1 in 10 women (10%)
• Loss of bone density caused by a lack of oestrogen over a long period of time – the bones are weaker and more likely to break. You will have a DEXA scan to check your bone density before you start treatment
• Higher level of cholesterol in the blood – this is usually only slightly raised and you will have regular blood tests to check it
• Diarrhoea – drink plenty of fluids and tell your doctor or nurse if diarrhoea becomes severe or continues for more than 3 days
• Dryness of the vagina
• Vaginal bleeding in fewer than 1 in 20 women (5%) – this mainly happens when women change from one type of hormone therapy to another and may last a few weeks. Tell your doctor or nurse if the bleeding continues
• Liver changes that are very mild and unlikely to cause symptoms – the liver will almost certainly go back to normal when treatment is finished, but you will have regular blood tests to check how well your liver is working

The side effects above may be mild or more severe. A side effect may get better or worse through your course of treatment. Or you may get more side effects as the course goes on. This depends on

• How many times you've had the drug before
• Your general health
• The amount of the drug you have (the dose)
• Other drugs you are having

Talk to your doctor, pharmacist or nurse about all your side effects so that they can help you manage them. You should have a contact number for your nurse. You can ring if you have any questions or problems. They can give you advice or reassure you. If in doubt, call them.

Tell your doctor about any other medicines you are taking, including vitamins, herbal supplements and other over the counter remedies – some drugs can react together.