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Anastrozole (Arimidex) is a hormone therapy that lowers the oestrogen level in the body. Find out more about when anastrozole is given, how it works and its side effects.

1. What is anastrozole?

Anastrozole is a hormone therapy drug used to treat breast cancer in women who have gone through a natural menopause (when periods stop).

It can also be used:

  • To treat breast cancer in premenopausal women having treatment to stop the ovaries working (ovarian suppression)
  • To reduce the risk of breast cancer in women who have an increased risk because of their family history
  • To treat men with breast cancer, although another drug called tamoxifen is more commonly used

You may also hear it called Arimidex, which is its best-known brand name. There are a number of other brands of anastrozole, all of which contain the same dose of the drug.

2. How anastrozole works

Anastrozole works by reducing the amount of oestrogen made in the body. It belongs to a group of drugs called

Some breast cancers use oestrogen in the body to help them to grow. These are known as oestrogen receptor positive or ER-positive breast cancers.

3. Who might be offered anastrozole?

Anastrozole is suitable for women who have been through the menopause and whose breast cancer is oestrogen receptor positive.

Sometimes anastrozole is given alongside ovarian suppression to women who haven’t yet been through the menopause. If your cancer is hormone receptor negative, then anastrozole will not be of any benefit.

Anastrozole may be given to reduce the risk of breast cancer in women who haven’t had breast cancer but have an increased risk of developing it because of their family history.

4. When is anastrozole given?

Anastrozole is usually given after surgery to reduce the risk of breast cancer coming back or spreading.

If you’re having or , your specialist will tell you when it’s best to start anastrozole. 

Occasionally, anastrozole may be used as the first treatment for breast cancer, for example when surgery isn’t appropriate or needs to be delayed. It’s sometimes given before surgery to shrink a large breast cancer.

Anastrozole can also be used to treat breast cancer that has come back (). It can also be given to treat breast cancer that has spread to another part of the body (), when it’s often given alongside another drug. 

5. How is anastrozole given?

Anastrozole is taken as a tablet once a day, 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. The level of drug in your body will remain high enough from the day before.

Some brands of anastrozole contain small amounts of lactose. If you know you are lactose intolerant discuss this with your treatment team or pharmacist.

6. How long is anastrozole given for?

The recommended length of time that anastrozole is taken for will depend on your individual situation.

People being treated for primary breast cancer will usually take anastrozole for 5 to 10 years.

Some people start taking anastrozole after a number of years of taking another hormone therapy drug called tamoxifen.

If you’re taking anastrozole to treat breast cancer that has come back or spread to another part of the body, you’ll usually take it for as long as it’s effective and any side effects can be managed.

­If you’re taking anastrozole to reduce the risk of breast cancer developing because of your family history, you’ll usually take it for 5 years.

7. Side effects of anastrozole

Like any drug, anastrozole can cause side effects. Everyone reacts differently to drugs and some people have more side effects than others.

8. Stopping anastrozole

Your treatment team will tell you when to stop taking anastrozole. You won’t need to stop taking it gradually. 

Some people worry about stopping their treatment, but there’s evidence that anastrozole continues to reduce the risk of breast cancer coming back for many years after you stop taking it.

However, not taking the drug for the recommended time may increase the risk of your breast cancer coming back. If you’re thinking about stopping taking anastrozole for any reason, talk to your specialist first. Sometimes it may be possible to change to another hormone therapy.

Hormone therapy is a very common treatment for secondary breast cancer and many people take it for a long time. If anastrozole stops working, your specialist may prescribe another hormone therapy drug. If you have any worries or questions about taking or stopping anastrozole, you can call us on our free helpline number below to talk through your concerns.

9. Other important information

Taking anastrozole with other drugs

If you’re taking any other prescribed or over-the-counter medicines, check with your treatment team or pharmacist if you can take these with anastrozole.

Do not take other drugs containing oestrogen, such as hormone replacement therapy (HRT), while you’re taking anastrozole as this may interfere with its effectiveness. 

Talk to your specialist, pharmacist or GP about any complementary therapies, herbal remedies or supplements you want to use before you start using them.

Sex and contraception

Taking anastrozole while pregnant may be harmful to a developing baby. It’s possible to become pregnant while taking anastrozole even if your periods have stopped with ovarian suppression.

If you’re sexually active with a chance of becoming pregnant, your specialist is likely to advise you to use a non-hormonal method of contraception while taking anastrozole. Non-hormonal methods include condoms, Femidoms or a diaphragm. It may also be possible to use a coil (IUD or intrauterine device). However, you would need to discuss this with your treatment team as not all types are suitable for women with breast cancer.

If you think you may be pregnant, stop anastrozole and tell your specialist as soon as possible.


The impact of anastrozole on fertility is not currently known. 

It’s important to discuss any fertility concerns with your treatment team before you begin your treatment.


Breastfeeding is not recommended while having anastrozole, or within a week of the last dose. This is because there’s a risk the drugs could be passed on through breast milk.

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Quality assurance

This information was published in April 2022. We will revise it in April 2024.

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