Tamoxifen is a drug used to treat breast cancer in both pre-menopausal women (women who have not yet gone through the menopause) and post-menopausal women (women who have gone through the menopause).

It can also be taken by men who have breast cancer.

It is a type of hormone treatment, also known as endocrine therapy.

How does it work?

Tamoxifen will only be prescribed if your breast cancer has receptors within the cell that bind to the hormone oestrogen (known as oestrogen receptor positive or ER+ breast cancer).

Tamoxifen works on the whole body (known as systemic treatment) and blocks the effects of oestrogen on the receptors. This helps to stop any breast cancer cells from growing.

When is tamoxifen prescribed?

Tamoxifen can be used for primary breast cancer as additional treatment following surgery, to reduce the risk of the cancer coming back and to reduce the risk of a new breast cancer developing. This is known as adjuvant therapy. If you are going to take tamoxifen as part of your treatment for primary breast cancer, your specialist will tell you when is best for you to do this.

For people diagnosed with ductal carcinoma in situ (DCIS) the role of tamoxifen is less clear. Your specialist will discuss whether or not they feel it would be of benefit to you.

Occasionally, tamoxifen may be used as the first treatment for breast cancer. This may be when surgery is not appropriate, or before surgery to shrink a large breast cancer so that breast-conserving surgery rather than a mastectomy may be possible.

Tamoxifen may also be used for breast cancer that has come back in the breast or surrounding area (local recurrence or locally advanced) or for people diagnosed with secondary breast cancer.

Tamoxifen may be an option for some people who have a high risk of developing breast cancer because of a significant family history. It is given to try to reduce the risk of breast cancer developing.

How is tamoxifen taken?

Tamoxifen is taken as a tablet. Occasionally it may be prescribed as a liquid for those who have difficulty swallowing. 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 one the next day. The level of the drug in your body will remain high enough from the previous day.

If, for any reason, you want to stop taking tamoxifen, talk to your specialist first. This is because not taking the drug for the recommended time may worsen your outlook.

How long will I have to take tamoxifen?

The recommended length of time that tamoxifen is taken will vary according to individual circumstances. People being treated for primary breast cancer will usually take tamoxifen for between five and 10 years.  

Some women who have gone through the menopause will change to a different hormone therapy known as an aromatase inhibitor. This can happen after two to three years of taking tamoxifen, or after you have taken tamoxifen for five years. Sometimes tamoxifen is recommended after taking an aromatase inhibitor.   

For younger women who are pre-menopausal, tamoxifen may be given alone or sometimes alongside ovarian suppression (treatment to stop the ovaries working).

If you are taking tamoxifen for breast cancer that has come back in the breast or surrounding area, or for secondary breast cancer, you will usually continue to take it for as long as it is keeping your cancer under control.

Possible side effects of tamoxifen

Everyone reacts differently to drugs. Some people will experience more side effects than others, while some people do not experience any side effects at all. The side effects of tamoxifen are listed below.

Tamoxifen is made by a number of different manufacturers. Some people report a change in the side effects they experience if they take tamoxifen made by a different manufacturer. This suggests that some people find tamoxifen produced by one manufacturer seems to suit them better than another.

You may want to discuss this with your specialist or GP, or ask the pharmacist who dispenses your prescriptions if they can supply you with tamoxifen from the manufacturer you prefer.  

Common side effects

Menopausal symptoms

The most common side effects that people have when taking tamoxifen are menopausal symptoms. These include:

  • hot flushes
  • night sweats and sleep disturbance
  • vaginal irritation (such as dryness and itching)
  • loss of sex drive (libido)
  • mood changes.

To find out more about coping with these side effects see our information on menopausal symptoms and sex and intimacy.

Effects on periods

Women who are still having regular periods may find that these change. For example, they may be lighter and/or irregular or may stop altogether. However, this can also be because the menopause has occurred naturally during this time or because other treatment such as chemotherapy has caused an earlier menopause.

Vaginal discharge is common when taking tamoxifen, but let your treatment team or GP know so that any infection can be ruled out. 

Effect on bones

If you are post-menopausal, tamoxifen slows down the process of bone loss, reducing the risk of osteoporosis (thinning of the bone). However, pre-menopausal women may be at risk of thinning of the bones when taking tamoxifen. This is unlikely to lead to osteoporosis unless treatment has been given to stop the ovaries from working as well.

Indigestion or mild nausea

This is usually most noticeable when starting tamoxifen. These symptoms usually improve or become easier to manage over time and may be helped by taking the tamoxifen with food.

Less common or rare side effects

Taking tamoxifen increases the risk of blood clots such as deep vein thrombosis (DVT). Tell your doctor straight away if you experience symptoms such as a swelling or pain in your leg. People with a DVT are at risk of developing a pulmonary embolism. This is when part of the blood clot breaks away and travels to the lung. If you experience shortness of breath, chest pain or cough up blood you need to see a doctor urgently.

If you need to have treatment or surgery that will reduce your mobility, you should discuss with your specialist whether you need to stop taking tamoxifen before and after this. Your specialist will let you know how long this will be for, but it’s likely to be a couple of weeks.

Find out more about blood clots.

Tamoxifen can also affect the lining of the uterus or womb (known as the endometrium), which may become thickened. If you have any unexpected vaginal bleeding or pain, tell your GP or specialist. Occasionally tamoxifen may cause polyps or ovarian cysts or very rarely, cancer of the uterus (womb).

Tamoxifen can occasionally cause changes to how the liver works. These changes are usually very mild and unlikely to cause any symptoms. Once you finish your treatment your liver will almost certainly go back to normal.

Other less common or rare side effects

  • eyesight problems
  • hair thinning/hair loss
  • increase in downy facial hair
  • weight gain
  • joint pains
  • tiredness
  • headaches

Side effects from tamoxifen are usually not severe however, for some people they can cause distress and disruption to everyday life. Talk to your breast care nurse, specialist or GP as there may be treatments that can help.

Side effects in men

Men can have similar side effects to those experienced by women, such as hot flushes and headaches. A number of small studies have also noted other side effects, such as decreased sex drive, though this recovered after tamoxifen treatment ended. Read more about breast cancer in men.

Fertility and pregnancy

While you are taking tamoxifen you will be advised not to get pregnant as it may harm a developing baby. Even if your periods stop while you are taking tamoxifen you could still get pregnant.

If you have any concerns about taking tamoxifen talk to your specialist team or call us on 0808 800 6000.

Last reviewed: May 2015
Next planned review begins 2017

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