This information is about a hormonal therapy called tamoxifen that is used to treat breast cancer. It should ideally be read with our general information about breast cancer or, if appropriate, secondary breast cancer.

Tamoxifen is an anti-oestrogen drug that was developed over 30 years ago. It is used widely to treat breast cancer and occasionally some other cancers.

Tamoxifen can also be used to treat or prevent side effects of breast tenderness and swelling in some men with prostate cancer. These side effects can happen as a result of their hormone treatment.

The way in which tamoxifen works is quite complicated and not yet fully understood, but its main function is as an anti-oestrogen drug.

Most breast cancers need supplies of the female hormone oestrogen to grow. Cancer cells have proteins called receptors on their surface that the sex hormones attach to. Cancers with oestrogen-receptors on the surface of their cells are called 'oestrogen-receptor-positive' (ER-positive) and tamoxifen is most effective against these cancers.

Under normal circumstances, when oestrogen comes into contact with the receptors, it fits into them and activates the cancer cells to divide so that the tumour grows.

Tamoxifen imitates the action of oestrogen and fits into the receptor but does not activate the cells to divide. The tamoxifen stays in place and stops oestrogen from reaching the cancer cells so that they either grow more slowly or stop growing altogether.

Tamoxifen can greatly reduce the chance of oestrogen-receptor-positive cancers coming back after surgery. It can also be used to shrink large tumours down before surgery so that they can be removed.

Your doctor will consider a number of different factors before they choose the most appropriate treatment for you.

A test may be done to find out whether your breast cancer cells have oestrogen-receptors (ER) and progesterone-receptors (PR). This helps your doctor to decide on treatment, as ER-positive cancers respond well to tamoxifen.

Tamoxifen can be used after surgery for early breast cancer, to reduce the risk of the cancer coming back. Giving treatment in this way is known as adjuvant therapy. In the UK, tamoxifen is very commonly used to treat early breast cancer and is usually given for five years.

Sometimes tamoxifen is used before surgery, to shrink a large breast cancer so that a lumpectomy (removal of the lump) is possible, rather than a mastectomy (removal of the whole breast). Tamoxifen given in this way is known as neo-adjuvant therapy.

Tamoxifen is also sometimes used to control or shrink a cancer that has come back after initial treatment or cannot be removed when it is first diagnosed. In this situation, it can often control a cancer for long periods of time.

Many breast cancers rely on supplies of the hormone oestrogen to grow. In women who have had their menopause, the main source of oestrogen is through changing androgens (sex hormones produced by the adrenal glands) into oestrogen. This is carried out by an enzyme called aromatase. The conversion process is known as aromatisation, which happens mainly in the fatty tissues of the body.

Drugs that block the process of aromatisation, and so reduce the amount of oestrogen in the body, are known as aromatase inhibitors. Aromatase inhibitors include anastrozole (Arimidex®), exemestane (Aromasin®) and letrozole (Femara®).

Research trials have looked into using aromatase inhibitors to treat early breast cancer. Early results from these trials indicate that using aromatase inhibitors, either instead of tamoxifen, or after it has been given, may give fewer side effects and possibly less chance of the cancer coming back than the standard five years of tamoxifen. However, it is unclear whether they will actually improve survival and there may be long-term effects from taking these drugs that we do not know about yet. Your doctor can discuss this with you further if you are concerned about which hormonal therapy you should be taking.

Research is currently being done to see if tamoxifen can prevent breast cancer in women who have a high risk of developing the disease. 'High risk' is defined as having one or more close relatives (mother, sister) who had breast cancer before they were 50. The results of some trials are available, but others are still ongoing. If you think that a member of your family may be at high risk of breast cancer, they should discuss the possible monitoring and treatment options with their doctor.

Tamoxifen is taken as a tablet that should be swallowed whole with a glass of water. The tablets come in different strengths: 10mg, 20mg and 40mg. The drug is also manufactured under different brand names and these may appear on the tablets. It is also available as a sugar-free syrup for people who have difficulty swallowing tablets.

Tamoxifen is usually prescribed as a single daily dose and this should be taken at the same time each day: the actual time does not matter. Some women prefer to take the tablet with food as it may make you feel sick and can leave a metallic taste in your mouth. It is best to find a convenient time and stick to it.

Trials are under way to determine how long, after surgery for breast cancer, tamoxifen should be taken for. It is commonly prescribed for five years, although some doctors prescribe it for two years and others indefinitely. Currently it is thought that five years of treatment is probably best. Aromatase inhibitors are sometimes given after two or three years of treatment with tamoxifen.

Each person's reaction to any medication is different. As people react to drugs in different ways, it is impossible to predict who is going to have side effects. Many women who take tamoxifen have no side effects, while others will experience them.

Very rarely, if the side effects are severe, you may have to stop taking tamoxifen and a different drug may be prescribed.

Side effects are more common in premenopausal women, who may develop menopausal side effects as a result of a lowered level of oestrogen. The most common side effects, apart from feeling sick (nausea), are hot flushes and sweats, particularly at night.

Flushes and sweats Sometimes the flushes will gradually lessen over the first few months but some women continue to have them for as long as they take tamoxifen. There are a number of ways to help to reduce or control hot flushes and sweats. Some women find it helpful to avoid or cut down on tea, coffee, nicotine and alcohol.

Some women find that complementary therapies can help, and your GP can give you details about obtaining these on the NHS. If you find your own complementary therapist, make sure that they are properly qualified and registered.

Sometimes other drugs can be prescribed to reduce hot flushes and sweats. Recent research suggests that progestogen or some types of antidepressant drugs may be helpful in controlling this unpleasant side effect. Your nurse or doctor can discuss this with you.

If you are having troublesome hot flushes you can discuss possible treatments with your doctor. Your doctor may be able to prescribe a different hormonal therapy instead. Sometimes the hot flushes continue for a long time after you have stopped taking tamoxifen.

Nausea and indigestion Feeling sick (nausea) and indigestion are fairly common but can often be relieved by taking your tablets with food or milk or at night. Although nausea is quite common initially, it usually wears off after a few weeks.

Weight gain Some women notice that they put on weight while taking tamoxifen. This may be due to water retention.

Change in periods Women who have not yet reached the menopause may notice that their monthly periods change – they may become irregular, lighter or sometimes stop altogether. Some women also notice an increase in vaginal discharge and itching of the area around the vagina (the vulva).

Depression, tiredness and dizziness Some women feel depressed while taking tamoxifen, but this may be due to other causes.

Headaches Some people affected by migraine notice a change in the pattern of their headaches.

Blood clots (Thrombosis) In postmenopausal women, tamoxifen can slightly increase the risk of blood clots and strokes. Pain, warmth, swelling or tenderness in an arm or leg (or any chest pain), must be reported to your doctor immediately.

Visual problems Blurred or reduced vision is very rare, but any visual changes should be reported to your doctor.

Voice changes This has been reported by some women. Professional singers may want to seek help and advice from their doctor.

Tumour flare Rarely, women who are prescribed tamoxifen for advanced cancer, where it has spread to the bones, may have tumour flare when they start taking tamoxifen. Tumour flare can cause a raised level of calcium in the blood (hypercalcaemia), with symptoms of nausea, vomiting and thirst. Very occasionally, a short stay in hospital is necessary until the calcium levels have been reduced.

Rare side effects of tamoxifen are mild allergic reactions which may include skin rashes and some hair falling out. This will regrow later.

Like most drugs, tamoxifen should not be taken during pregnancy. Even though it can affect a woman's periods, tamoxifen is not a contraceptive, so reliable contraception must be used.

If you have a history of blood clots or deep vein thrombosis (DVT) let your doctor know, as tamoxifen may not be suitable for you.

Tamoxifen can increase the effect of the drug warfarin, which is used to thin the blood in people prone to blood clots. If you are taking warfarin, let your doctor know straight away.

Studies have shown that post-menopausal women who take tamoxifen over a long period of time may have a very slightly increased risk of developing cancer of the lining of the womb (endometrial cancer).

However, this risk has to be weighed against the benefits of tamoxifen in treating the breast cancer, which generally far outweigh this small risk.

If detected early, treatment for endometrial cancer is usually very successful. Early warning signs are abnormal vaginal bleeding, although this is often caused by non-cancerous conditions such as polyps. If you have any abnormal vaginal bleeding you should let your doctor know.

In some cancer hospitals women are given regular gynaecological check-ups to detect early signs of endometrial cancer. An ultrasound scan, using sound waves, may be done to check for signs of change in the womb lining. A small probe is inserted into the vagina and the doctor can look at the scan on a screen. Any changes can be seen straight away. The scan is safe and only takes a few minutes.

For most women the beneficial effects of tamoxifen far outweigh the risks.

Is it true that tamoxifen can protect against heart disease?
There is some evidence suggesting that while you are taking tamoxifen it can lower the level of fat (lipids) in your blood (high levels of fat contribute to heart disease). It is also thought that tamoxifen may help to prevent bone loss and so may reduce the risk of osteoporosis (thinning of the bone) in some women.

Is tamoxifen the only hormonal drug used to treat breast cancer?
No, many other types of hormonal therapy drugs are used – for example, drugs called aromatase inhibitors which block the production of oestrogen.

If you take a drug for a long time, it is possible for the cancer cells to build up a resistance to it, so different drugs might then have to be used.

Is it possible to become pregnant after taking tamoxifen?
Yes, it is possible for some women. But if you are considering becoming pregnant it is best to get advice from your doctor first.

Tamoxifen may interact with other medicines. Let your doctor know about any medications you are taking, including non-prescribed drugs such as complementary therapies and herbal drugs.

• Keep the tablets in a safe place where children cannot reach them, as tamoxifen could harm them.
• Store the tablets in the original packaging/container. They should be kept at room temperature, away from moisture and direct heat.
• If your doctor decides to stop the treatment, return any remaining tablets to the pharmacist. Do not flush them down the toilet or throw them away.
• If you are sick just after taking the tablet tell your doctor or nurse, as you may need to take another one.
• If you forget to take your tablet, don't worry. 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. Let your doctor or nurse know.
• Remember to get a new prescription a few weeks before you run out of tablets. Make sure you have plenty for holidays.

This section is based upon our Tamoxifen factsheet which has been compiled using information from a number of reliable sources, including:

• Martindale: The Complete Drug Reference (35th edition). Sweetman et al. Pharmaceutical Press, 2007.
• British National Formulary (54th edition). British Medical Association and Royal Pharmaceutical Society of Great Britain, September 2007.

For further references, please see the general bibliography.