This factsheet explains briefly what tamoxifen is, how it works, when it’s prescribed and what side effects may occur.
What is tamoxifen?
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.
Tamoxifen will only be prescribed if your breast cancer has receptors within the cell that bind to the female hormone oestrogen and stimulate the cancer to grow (known as oestrogen receptor positive or ER+ breast cancer). All breast cancers are tested for oestrogen receptors using tissue from a biopsy or after surgery.
If your cancer is oestrogen receptor positive it means that tamoxifen may be appropriate for you. Tamoxifen works on the whole body (known as systemic treatment) by blocking the effects of oestrogen on breast cancer cel s, stopping them from growing.
When oestrogen receptors are not found (oestrogen receptor negative or ER-) tests may be done for progesterone (another female hormone) receptors. As oestrogen receptors play a more important role than progesterone receptors, the benefits of hormone therapy are less clear for people whose breast cancer is only progesterone receptor positive (PR+ and ER-). If this is the case your specialist will discuss with you whether or not tamoxifen is appropriate.
If your cancer is found to be hormone receptor negative, then tamoxifen will not be of any benefit to you.
When is tamoxifen prescribed?
Tamoxifen can be used for primary breast cancer (breast cancer that has not spread beyond the breast or the lymph nodes (glands) under the arm) as additional treatment fol owing surgery, to reduce the risk of the cancer coming back. This is known as adjuvant therapy. Because each individual situation is different, if you are going to take tamoxifen as part of your treatment for primary breast cancer, your specialist will discuss when is best for you to do this.
Very occasional y, 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 (usual y referred to as wide local excision or lumpectomy) may be possible, rather than a mastectomy (removal of the whole breast).
Tamoxifen may also be used for secondary breast cancer (when cancer cel s from the breast have spread to other parts of the body) to control and slow down the spread of the disease.
Clinical trials are ongoing to look further into the use of tamoxifen particularly for certain groups of people, including those diagnosed with an early form of breast cancer cal ed ductal carcinoma in situ (DCIS) and those who have a high risk of developing breast cancer because of a significant family history.
How is tamoxifen taken?
Tamoxifen is taken either as a tablet or a liquid. The recommended dose for most people is 20mg daily. 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. Don’t stop taking tamoxifen without talking to your specialist first.
How long will I have to take tamoxifen?
People being treated for primary breast cancer will usual y take tamoxifen for up to five years, although this time can vary according to individual circumstances. There is ongoing research looking at the length of time that treatment with tamoxifen should continue for.
Some women who have gone through the menopause are changed to a different hormone therapy known as an aromatase inhibitor, such as anastrozole, exemestane or letrozole. This can happen after two to three years of taking tamoxifen, or after you have taken tamoxifen for five years.
For more information, see our Anastrozole (Arimidex)
and Letrozole (Femara)
4 | How long will I have to take tamoxifen?
For younger women who are pre-menopausal, tamoxifen may be given alone or alongside ovarian suppression (treatment to stop the ovaries working). For more information about ovarian suppression please see our
Ovarian ablation and suppression
For people being treated for secondary breast cancer, tamoxifen will be given for as long as it is helping to keep the cancer under control.
What are the 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 al . The side effects of tamoxifen are listed below.
Tamoxifen is made by a number of different manufacturers. Some of the tablets may differ in their additional ingredients (for example, preservatives). This means that the colour, shape, size and sometimes taste of the tablets can differ between brands. However, this does not alter the effectiveness of the treatment.
Some people report a change in the side effects they experience if they take tamoxifen produced by a different manufacturer. This suggests that some people find tamoxifen produced by one manufacturer seems to suit them better than another.
If this happens, you may wish to discuss this with your specialist or ask the pharmacist who dispenses your prescriptions if they can supply you with tamoxifen from the manufacturer you feel most comfortable with.
The most common side effects are similar to menopausal symptoms,
including hot flushes, night sweats and sleep disturbance, vaginal
irritation (such as dryness, itching or discharge), loss of sex drive (libido)
and mood changes. To find out more about coping with menopausal
symptoms see our Menopausal symptoms and breast cancer
Some people experience indigestion or mild nausea. This is usual y most noticeable when they start taking tamoxifen. These symptoms usual y improve or become easier to manage over time and may be helped by taking the tamoxifen with food.
Women who are still having regular periods may find that they become irregular, lighter or stop completely. However, this may also be because the menopause has occurred natural y during this time or because other treatment such as chemotherapy has caused an earlier menopause.
If you are post-menopausal there is evidence that 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. But this would be unlikely to lead to osteoporosis unless treatment has been given to stop the ovaries from working as wel .
If you have concerns about this, you may want to discuss it with your
specialist. For more information see our factsheet Breast cancer
treatment and the risk of osteoporosis
A small number of women notice an increase in downy facial hair or changes to their singing voice. Some women find they experience some hair loss or hair thinning while taking tamoxifen.
Taking tamoxifen also increases the risk of blood clots such as deep vein thrombosis (DVT). Tell your doctor straight away if you experience any symptoms such as a swel ing or pain in your leg, or a feeling of shortness of breath.
If you have had blood clots in the past or are at risk of developing them, your specialist will take this into account and may use an alternative treatment. If you take an anti-coagulant (medicine which reduces the ability of the blood to clot), such as warfarin, then your doctor may want to monitor you more regularly.
For anyone taking tamoxifen it’s important to be aware of the risk of blood clots and ways you can help reduce this. Long periods of inactivity can increase your risk of developing blood clots. If you’re planning a long-distance plane, car or train journey, it is a good idea to get advice from your specialist or GP.
Additional y, if for any reason you need to have treatment or surgery that will reduce your mobility, you should discuss with your specialist whether you will need to stop taking tamoxifen before this.
6 | What are the possible side effects of tamoxifen?
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. In a very few cases, prolonged use of tamoxifen may cause polyps or ovarian cysts or, even more rarely, cancer of the uterus (womb).
There is a very slight risk of changes to your vision. If you notice this, tel your GP or specialist.
Other reported side effects include joint pains, tiredness, difficulty concentrating, headaches and leg cramps at night.
Some women find they put on weight during treatment, although there is no clear evidence linking weight gain to tamoxifen.
Side effects from tamoxifen are usual y not severe and it is uncommon for someone to stop taking tamoxifen because of side effects. However, for some people side effects can be severe and long lasting, causing distress and disruption to everyday life. Talk to your breast care nurse, specialist or GP (local doctor) as there may be treatments that can help.
Are there different side effects for 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.
Can I take tamoxifen with other drugs?
Always check with your specialist if you are taking any other medicines with tamoxifen.
A few studies have suggested that some drugs like fluoxetine (Prozac), paroxetine (Seroxat), bupropion (Wel butrin, Zyban, Voxra, Budeprion or Aplenzin), quinidine (Quinidine Gluconate or Quinidine Sulfate), cinacalcet (Sensipar®) may interfere with the way tamoxifen works, making it less effective. More recent research suggests this might not be the case, but it may be recommended that these drugs are not used in conjunction with tamoxifen.
Many women consider taking herbal medicines while having treatment for breast cancer. However, these should be used with care. Herbal preparations and hormonal treatments to help with menopausal
symptoms may be unsuitable when you have an oestrogen receptor positive breast cancer.
The exact effect and safety of many herbal preparations and hormonal treatments after a diagnosis of breast cancer is not ful y understood and you should ask for advice from your specialist before considering taking any of these.
Fertility and pregnancy
In most pre-menopausal women who take tamoxifen the ovaries continue to work. When you start taking tamoxifen it may stimulate ovulation (release of the egg from the ovary) making you more fertile. With continued use, in some women (approximately half) periods become less regular, lighter or stop altogether. General y your periods will start again once you stop taking tamoxifen (as long as you have not gone through the menopause natural y while taking the drug). However, it may take four to five months for your cycle to become regular again.
Tamoxifen is usual y taken for five years and you will be advised not to get pregnant while taking it as it may harm a developing baby. Even if your periods stop while you are taking tamoxifen you could still get pregnant. This means that if you are sexual y active with a man while taking tamoxifen, your specialist is likely to advise you to use a non-hormonal or effective barrier method of contraception. If you think you may be pregnant, tell your specialist as soon as possible.
Because of the length of time it’s taken for, the side effects of tamoxifen may mask the onset of a natural menopause. It may only be when you finish taking it that you realise you have started your menopause. If you want to have children and you are in your late 30s or early 40s, then taking tamoxifen for as long as five years may be an issue you want to discuss with your specialist team.
For more information see our publication Fertility issues and breast
If you are planning to get pregnant after you have finished taking tamoxifen, it is advisable to wait at least two months to al ow time for the drug to leave the body completely. You should talk to your specialist team about the most appropriate length of time for you.
If you have any concerns about taking tamoxifen, you can talk to your
specialist or breast care nurse. You may also find it helpful to talk to
someone who has had a similar experience to you. You can do this one
to one, or in a support group. For more information on individual support
or support groups in your area, call our Helpline on 0808 800 6000
) or see the breast cancer services map on our website at
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For more information see our publications on:Anastrozole (Arimidex) BCC31
Exemestane (Aromasin) BCC46
Letrozole (Femara) BCC64
Ovarian ablation and suppression BCC27
Fertility issues and breast cancer treatment BCC28
Menopausal symptoms and breast cancer BCC18
Complementary therapies BCC55
Breast cancer treatment and the risk of osteoporosis BCC75
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