Tamoxifen is an anti-oestrogen drug given to pre-menopausal and post-menopausal women and men with primary breast cancer. It can also be given to people with local or regional recurrence or secondary breast cancer. It’s a type of hormone treatment, also known as endocrine therapy.

Tamoxifen is only prescribed to people whose breast cancer is oestrogen receptor positive (ER+). It works on the whole body by blocking the effects of oestrogen on breast cancer cells, stopping them from growing.

When it’s prescribed

It’s prescribed to reduce the risk of the cancer coming back. This is known as adjuvant treatment (given in addition to other treatment, for example chemotherapy or radiotherapy as well as surgery).

Occasionally, it may be used as the first treatment for breast cancer if surgery is not appropriate, or to shrink a large breast cancer before surgery.

Taking it

Tamoxifen is taken as a tablet or liquid. The recommended dose is usually 20mg daily.

People with primary breast cancer usually take tamoxifen for up to five years. But there is ongoing research looking at the length of time for which treatment with tamoxifen should continue. Some women who have been through the menopause are changed to a different hormone therapy (called an aromatase inhibitor) after two to three years of tamoxifen, or after taking it for five years.

Pre-menopausal women may be given tamoxifen alone or alongside ovarian suppression (treatment to stop the ovaries working). People being treated for secondary breast cancer will be given it for as long as it helps to keep the cancer under control.

Side effects

Tamoxifen is made by various companies. Some tablets may differ in their additional ingredients (for example preservatives) but this doesn’t alter the effectiveness of the treatment.

Some people report a change in side effects if they take tamoxifen produced by a different manufacturer. If this happens, you can ask your specialist or the pharmacist if they can prescribe the brand of tamoxifen you prefer.

Everyone reacts differently to drugs and some people will experience more side effects than others. Men can experience similar side effects to women.

Common side effects

The most common side effects are similar to menopausal symptoms such as:

  • hot flushes
  • night sweats
  • loss of libido (sex drive)
  • vaginal dryness
  • mood changes.

To find out more, see our information on menopausal symptoms.

In most pre-menopausal women who take tamoxifen the ovaries continue to work and, at first, may stimulate ovulation, making you more fertile. You will be advised not to get pregnant while taking tamoxifen and to use contraception.

Over time, pre-menopausal women may find their periods become irregular, lighter or stop completely.

Generally your periods will return once you stop taking tamoxifen unless you go through the menopause naturally while taking it or chemotherapy has caused an early menopause.

If you are planning to get pregnant after you have finished tamoxifen, you may want to discuss this with your specialist.

If you are post-menopausal, tamoxifen slows down the natural 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 would be unlikely to lead to osteoporosis unless treatment has been given to stop the ovaries from working as well.

Other reported side effects include:

  • mild indigestion and nausea
  • leg cramps
  • joint pains
  • headaches.

Less common or rare side effects include:

  • eyesight problems
  • hair thinning/hair loss
  • increase in downy facial hair
  • weight gain.

Tamoxifen can also affect the lining of the womb (endometrium) which may thicken. Occasionally, tamoxifen can cause polyps or ovarian cysts, and very rarely may cause cancer of the womb.

Tamoxifen also increases the risk of blood clots, so report symptoms such as swelling, pain in your leg or shortness of breath straight away.

If you experience any side effects, talk to your breast care nurse, specialist or GP (local doctor) as there may be treatments that can help.

Content last reviewed March 2013; next planned review 2015