Anastrozole (Arimidex ®)
This information is about a hormonal therapy used to treat breast cancer called anastrozole, which is also called Arimidex ®. Throughout this page we refer to it by its more commonly used name, Arimidex.
This information should ideally be read with our general information about breast cancer or secondary breast cancer.
You will see your doctor regularly while you have this treatment so they can monitor its effects. This information should help you discuss any queries about your treatment and its side effects with your doctor or specialist nurse.
Arimidex is a hormonal therapy used to treat breast cancer in women who have been through the menopause.
Hormones are substances produced naturally in the body. They act as chemical messengers and help control the activity of cells and organs. Hormonal therapies work by interfering with the production or action of particular hormones in the body.
Some breast cancer cells have receptors, which allow particular types of hormones to attach to the cancer cell. In order to grow, many breast cancers rely on the hormone oestrogen. These cancers are known as oestrogen receptor-positive or ER-positive breast cancers.
In women who have had their menopause, the main source of oestrogen is through the conversion of androgens (sex hormones produced by the adrenal glands) into oestrogens. This is carried out by an enzyme called aromatase. The conversion process is known as aromatisation, and it happens mainly in the fatty tissues of the body.
Arimidex is a drug that blocks the process of aromatisation, and so reduces the amount of oestrogen in the body. As less oestrogen reaches the cancer cells, they grow more slowly or stop growing altogether. Drugs that work in this way are known as aromatase inhibitors. Other aromatase inhibitors include letrozole (Femara ®) and exemestane (Aromasin ®).
Arimidex is a tablet that is taken once a day. It should be swallowed whole with a glass of water, at about the same time each day. It doesn't matter whether this is in the morning or evening.
Your doctor will take into account a number of different factors when planning your treatment. Arimidex is used to treat post-menopausal women with oestrogen receptor-positive breast cancer.
Early breast cancer
Arimidex may be given to women with early breast cancer (cancer that has not spread) after they have had surgery to remove the cancer. In this situation, Arimidex is given to reduce the risk of cancer coming back. Giving treatment after surgery to reduce the chance of recurrence is known as adjuvant therapy.
For some women, Arimidex may be more effective than tamoxifen (another hormonal therapy commonly used for breast cancer), and it has different side effects. Studies show that switching to Arimidex after taking tamoxifen for 2-3 years may be better for some women than taking tamoxifen for five years.
Advanced breast cancer
Arimidex can be used to treat women who have breast cancer that has spread to other parts of the body (metastatic or secondary breast cancer). It can also be used to treat women whose breast cancer has come back after initial treatment.
You might find our information about staging and grading of breast cancer useful.
Your doctors will discuss with you the length of treatment they feel is appropriate for you. Arimidex may be given over a number of years, or for as long as it is controlling your cancer, depending on your individual situation.
Each person's reaction to any medicine is different. Most people have very few side effects with Arimidex, while others may experience more. The side effects described here won't affect everyone and may be different if you are taking more than one drug.
We have outlined the most common side effects, but haven't included those that are rare and therefore unlikely to affect you. If you notice any effects that aren't listed here, discuss them with your doctor or nurse.
You may have some of the following side effects, to varying degrees:
Hot flushes and sweats
These are usually mild and may wear off after a period of time. Sometimes people find it helps to cut down on tea, coffee, nicotine and alcohol. Research suggests that hormones called progestogens or some types of antidepressants may be helpful in controlling this side effect. Your doctor or nurse can discuss this with you.
Some people find complementary therapies such as acupuncture helpful. Your GP may be able to give you details about having these on the NHS. If you find your own therapist, make sure that they are properly qualified and registered. We have information about finding a therapist.
You can read more about treatments for menopausal symptoms like hot flushes in our information about managing menopausal symptoms.
Risk of osteoporosis
Women who have, or are at risk of, osteoporosis (weakened bones), should have their bone strength assessed before and during treatment with Arimidex. Some women may need to take bone-strengthening drugs to help prevent osteoporosis developing.
This may occur while using Arimidex. Gels that can help to overcome the dryness are available. You can buy these from a chemist or your doctor can prescribe them.
Arimidex can cause skin rashes. It’s important to let your doctor know if this happens. Your doctor can prescribe medicine to help with this.
Feeling sick (nausea) and being sick (vomiting)
These side effects are rare and usually mild. They can usually be effectively treated, so let your doctor know if you are affected. Feeling sick can often be relieved by taking your Arimidex tablet with food or at night.
This can usually be easily controlled with medicine, but tell your doctor if it is severe or continues. It's important to drink plenty of fluids if you have diarrhoea.
Some women notice that their hair becomes thinner while taking Arimidex. This is usually mild and the hair grows back at the end of treatment.
Some people have headaches while taking Arimidex, but this is not common. It's important to drink plenty of fluids. Let your doctor know if you are getting headaches, as they can prescribe medication to help.
Some women have some vaginal bleeding, usually in the first few weeks of treatment. This is rare and usually occurs after changing from other hormonal therapies to treatment with Arimidex. If the bleeding continues, tell your doctor or breast care nurse.
Joint pains/muscular stiffness
Some women have pain and stiffness in their joints while taking Arimidex. Let your doctor know if these effects are a problem. You may find it helpful to take mild painkillers.
Tiredness (fatigue) and lethargy
Some people can have increased tiredness, especially at the start of treatment. It's important to get plenty of rest. If you feel sleepy you should not drive or operate machinery.
Always let your doctor or nurse know about any side effects you have. There are usually ways in which they can be controlled or improved.
Things to remember about Arimidex tablets
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Arimidex may interact with other medicines. Tell your doctor about any medicines you are taking, including non-prescribed drugs such as complementary therapies, vitamins and herbal drugs.
Keep the tablets in a safe place, out of the reach of children.
Keep tablets in the original packaging and store them at room temperature, away from heat and direct sunlight.
Don't worry if you forget to take your tablet. Do not take a double dose. The levels of the drug in your blood will not change very much, but try not to miss more than one or two tablets in a row.
If your doctor decides to stop the treatment, return any remaining tablets to the pharmacist. Don't flush them down the toilet or throw them away.
Remember to get a new prescription a few weeks before you run out of tablets and make sure that you have plenty for holidays.
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Non-cancer hospital admission
If you’re admitted to hospital for a reason not related to the cancer, it’s important to tell the doctors and nurses looking after you that you are having hormonal treatment. You should tell them the name of your cancer specialist so that they can ask for advice.
It’s a good idea to know who you should contact if you have any problems or troublesome side effects when you’re at home. You can ask your doctor or nurse about this if you’re not sure.
This information is based on our Anastrozole (Arimidex®) fact sheet and has been compiled using information from a number of reliable sources, including:
British National Formulary. 63rd edition. 2012. British Medical Association and Royal Pharmaceutical Society of Great Britain.
Early and locally advanced breast cancer: diagnosis and treatment. February 2009. National Institute for Health and Clinical Excellence (NICE).
electronic Medicines Compendium (eMC). www.medicines.org.uk (accessed September 2012).
Sweetman, et al. Martindale: The Complete Drug Reference. 37th edition. 2011. Pharmaceutical Press.
Thanks to Debbie Wright, Oncology Pharmacist, Jane Depledge, Senior Lecturer, and all the people affected by cancer who reviewed this information. Reviewing information is just one of the ways you could help when you join our Cancer Voices network.