Anastrozole (Arimidex)

Anastrozole is a type of hormone therapy. You may also hear anastrozole called by its brand name Arimidex.

It’s used to treat breast cancer in women who have been through a natural menopause.

It can also be used in women who haven’t been through the menopause who are having treatment to stop their ovaries working.

Men with breast cancer may take anastrozole, although another drug called tamoxifen is more commonly used.

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

Before the menopause, oestrogen is mainly made in the ovaries. The ovaries no longer make oestrogen after the menopause but some oestrogen is still made in other tissues. This is mainly in body fat and involves a type of protein (enzyme) called aromatase.

Anastrozole belongs to a group of drugs called aromatase inhibitors, which stop the aromatase enzymes from working. This means there’s less oestrogen in the body to help breast cancer cells grow.

Primary breast cancer

You may be offered anastrozole if you have primary breast cancer.

You usually take anastrozole after surgery to reduce the risk of breast cancer returning in the same breast or spreading somewhere else in the body. This is known as adjuvant (additional) therapy.

Occasionally, anastrozole may be the first treatment for breast cancer. This may be when surgery is not appropriate or needs to be delayed. It’s sometimes given before surgery to shrink a larger breast cancer (known as neo-adjuvant or primary therapy).

Breast cancer that has come back or spread

You may be offered anastrozole if you have:

• Local recurrence
• Locally advanced breast cancer 
• Metastatic (secondary) breast cancer

To reduce the risk of breast cancer developing

You may be given anastrozole if you do not have breast cancer but have a high risk of developing it because of your family history.

It’s given to try to reduce the risk of breast cancer developing.

Alongside ovarian suppression

Anastrozole is sometimes given alongside ovarian suppression if you haven’t been through the menopause.

You take anastrozole as a tablet once a day, with or without food. You should take it at the same time every day.

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

What happens if I miss a dose? 

If you miss a dose of anastrozole, you do not need to take an extra dose the next day. The level of drug in your body will remain high enough from the day before.

How long you take anastrozole will depend on your individual situation.

Primary breast cancer 

If you have primary breast cancer, you’ll usually take anastrozole for 5 to 10 years.

You may start taking anastrozole after having taken a hormone therapy drug called tamoxifen for a number of years.

Breast cancer that has come back or spread

For breast cancer that has come back or spread, you’ll have anastrozole for as long as your treatment team feels you’re benefitting from it and any side effects are manageable.

To reduce the risk of breast cancer developing

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.

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

Primary breast cancer 

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 after 5 to 10 years. 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 anastrozole for the recommended time may increase the risk of your breast cancer coming back.

If you’re thinking about stopping anastrozole for any reason, talk to your treatment team first. It may be possible to change to another hormone therapy.

If you’re worried about any long-term side effects after stopping anastrozole, speak to your treatment team or GP.

Metastatic (secondary) breast cancer

Hormone therapy is a very common treatment for metastatic breast cancer. You’ll have anastrozole for as long as your treatment team feels you’re benefitting from the drug and the side effects are manageable. If anastrozole stops working, your treatment team may prescribe another hormone therapy drug.

Blood clots

People with breast cancer have a higher risk of blood clots such as a DVT (deep vein thrombosis). If you have a DVT, there’s a risk part of the blood clot could break away and travel to the lung. This is known as a pulmonary embolism (PE).

Blood clots can be life-threatening and should be treated quickly.

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.

Herbal medicines and supplements

Many people consider taking herbal products or supplements while having treatment for breast cancer. However, the exact effect and safety of taking these after a diagnosis of breast cancer is not fully understood.

Ask your treatment team or pharmacist before taking herbal products or supplements.

You can read more about supplements in our information on healthy eating after breast cancer treatment.

Sex, contraception and pregnancy 

You’re advised not to become pregnant while having treatment because anastrozole can harm a developing baby. If you have not been through the menopause, talk to your team about the most suitable method of contraception for you. It’s still possible to become pregnant even if your periods are irregular or have stopped naturally or because of treatment for breast cancer.  

Fertility

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.

If you’re planning to get pregnant after you have finished taking anastrozole, or want to take a break from anastrozole to become pregnant, speak to your treatment team.

Breastfeeding

Breastfeeding is not recommended while having anastrozole. This is because there’s a risk the drugs could be passed on through breast milk.

If you have any worries or questions about anastrozole, you can call our helpline free on 0808 800 6000 to talk through your concerns. You can also email nurse@breastcancernow.org

You can find out more about our support services below.