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Anastrozole (Arimidex)

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This page tells you about the hormone therapy anastrozole and its possible side effects. There are sections about


What anastrozole is

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


How anastrozole works

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.


How you have anastrozole

You take anastrozole as a tablet, once a day. Try to take it at the same time each day. Swallow the tablet whole with a drink of water. You can take it before, with, or after food.

It is very important that you take tablets according to the instructions your doctor or pharmacist gives you. You should take the right dose, not more or less. It is a long term treatment and you usually need to take it for several years. Never stop taking a cancer drug without talking to your specialist first.

If you accidentally take more anastrozole than you should, let your doctor know straight away. 

If you forget a dose, continue with the next dose as normal. Don't take an extra dose to make up for the missed one. 

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 you can look at the cancer drug side effects section.


Common side effects

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

  • Headaches
  • Hot flushes and sweats
  • Feeling sick is usually mild and can usually be controlled by anti sickness medicines
  • Skin rashes – these are usually mild but let your doctor or nurse know if you have a rash
  • Painful or stiff joints happen in more than 3 out of 10 women (30%)
  • Swollen joints (arthritis)
  • Feeling weak
  • 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
  • Mood changes happen in up to 2 in 10 women (20%)
  • Tiredness or fatigue happens in up to 2 out of 10 women (20%) 
  • A lowered interest in sex (reduced libido) occurs in more than 1 out of 10 women (10%)

Occasional side effects

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

  • 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
  • Raised cholesterol levels in the blood – this is usually only slightly raised and you will have regular blood tests to check it
  • Feeling sleepy
  • Being sick – you can have medicines to control this
  • Liver changes that are very mild and  unlikely to cause symptoms – they will usually go back to normal when treatment is finished, but you will have regular blood tests to check how well your liver is working
  • 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 – you can get vaginal moisturisers (lubricants) from your nurse or from the pharmacist
  • Vaginal bleeding occurs in fewer than 1 in 20 women (5%) – this mainly happens in the first few weeks of treatment. Tell your doctor or nurse if the bleeding continues

Rare side effects

Fewer than 1 in 100 people have these effects.

  • Inflammation of the liver (hepatitis)
  • Trigger finger – the finger or thumb is fixed in a bent position. Tel your doctor or nurse if you notice this

Important points to remember

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

Coping with side effects

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

Other medicines

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

Pregnancy and contraception

This drug may harm a baby developing in the womb. It is important not to become pregnant or father a child while you are having treatment with this drug and for a few months afterwards. Talk to your doctor or nurse about effective contraception before starting treatment.


Do not breastfeed during this treatment because the drug may come through in the breast milk.

Lactose intolerance

Anastrozole tablets contain lactose (milk sugar). If you have an intolerance to lactose, contact your doctor before taking this medicine.


More information about anastrazole

This page does not list all the very rare side effects of this treatment that are very unlikely to affect you. For further information look at the Electronic Medicines Compendium website at

If you have a side effect not mentioned here that you think may be due to this treatment you can report it to the Medicines Health and Regulatory Authority (MHRA) at

Arimidex may sometimes be used to treat breast cancer in men. Breast cancer is very rare in men. And so very little is known about the side effects of this drug in men. Doctors expect the effects to be similar to those listed on this page.

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Updated: 22 January 2014