This information is about a hormonal therapy used to treat breast cancer called tamoxifen. It describes tamoxifen, how it is given and some of its possible side effects.
This section should ideally be read with our general information about breast cancer or, if appropriate, secondary breast cancer.
Read our press release responding to new research on Tamoxifen.
You can also visit the Online Community to read our blog about Tamoxifen and discuss your thoughts with others.
Tamoxifen is an anti-oestrogen drug that is widely used to treat breast cancer. It‘s occasionally used to treat some other cancers.
Tamoxifen can also be used to treat or prevent side effects like breast tenderness and swelling in some men with prostate cancer. These side effects can happen as a result of other hormonal treatments.
Tamoxifen works in a way that’s quite complicated and not yet fully understood. Its main function is as an anti-oestrogen drug.
Many breast cancers rely on the female sex hormone oestrogen to grow. These cancers are known as oestrogen-receptor positive (ER-positive) breast cancers. Hormone-positive breast cancer cells have proteins called receptors, which sex hormones attach to.
When oestrogen comes into contact with the receptors, it fits into them and stimulates the cancer cells to divide so that the tumour grows. Tamoxifen fits into the oestrogen receptor and blocks oestrogen from reaching the cancer cells. This means the cancer either grows more slowly or stops growing altogether.
How tamoxifen is taken
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Tamoxifen is 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 manufactured under several different brand names and these may appear on the tablets.
It’s also available as a sugar-free syrup for people who have difficulty swallowing tablets, but you may need to check whether this is available with your pharmacist.
Tamoxifen is usually prescribed as a single daily dose, and this should be taken at the same time each day. You may prefer to take the tablet with food, as it may make you feel sick and can leave a metallic taste in your mouth. It's best to find a convenient time for you and stick to it.
When tamoxifen is given
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Your doctor will take into account a number of different factors when planning your treatment. The breast cancer cells will be tested to see whether they have oestrogen receptors (ER) and progesterone receptors (PR) present. Tamoxifen is only effective for people who have hormone-positive breast cancers.
Tamoxifen may be given after surgery for early breast cancer, to reduce the risk of the cancer coming back. This is known as adjuvant therapy.
You may be given tamoxifen for five years. Some people may have tamoxifen for two or three years and then change to a different type of hormonal therapy known as an aromatase inhibitor. Aromatase inhibitors include anastrozole (Arimidex ®), exemestane (Aromasin ®) and letrozole (Femara ®).
Occasionally, tamoxifen is used before surgery to shrink a large breast cancer. This means that a lumpectomy (removal of the lump) may be possible, rather than removal of the whole breast (a mastectomy).
Tamoxifen can also be used to control a cancer that has come back or spread to other parts of the body.
Each person's reaction to any medicine is different. Some people have very few side effects, while others may experience more. Very rarely, if the side effects are severe, you may have to stop taking tamoxifen and a different drug may be prescribed.
You may have some of the following side effects, to varying degrees:
Hot flushes and sweats
These are a common side effect of tamoxifen. They may gradually lessen over the first few months, but some people continue to have them for as long as they take tamoxifen.
There are a number of ways to help reduce or control hot flushes and sweats. Some people find it helps to avoid or cut down on tea, coffee, nicotine and alcohol. Research suggests that some types of antidepressant may be helpful in controlling this side effect. Your doctor or nurse can discuss this with you.
Some people may 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 complementary therapist, make sure that they are properly qualified and registered.
If the hot flushes are really troublesome, you can discuss possible treatments with your doctor. They 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.
You can read more about treatments for menopausal symptoms like hot flushes in our section on breast cancer and menopausal symptoms.
Feeling sick (nausea) and indigestion
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 improves after a few weeks.
Some people notice that they put on weight while taking tamoxifen. This may be due to water retention.
Change in periods
Women who have not yet had 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).
Some people get leg cramps with tamoxifen. Walking may stretch the muscle and help with this. Let your doctor know if leg cramps are a problem. If your leg becomes red, hot or swollen, tell your doctor immediately.
Less common side effects
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Depression, tiredness and dizziness
Some people feel depressed while taking tamoxifen, but this may be due to other causes.
Some people affected by migraines 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. If you have any pain, warmth, swelling or tenderness in an arm or leg (or any chest pain), you must tell your doctor straight away.
Blurred or reduced vision is very rare, but any changes in your eyesight should be reported to your doctor.
This side effect has been reported by some people. Professional singers may want to seek help and advice from their doctor.
People who are prescribed tamoxifen for cancer that has spread to the bones may experience something called tumour flare. This can cause a raised level of calcium in the blood (hypercalcaemia), with symptoms of nausea, vomiting and thirst. Occasionally, a short stay in hospital is needed until the calcium levels reduce.
Rare side effects of tamoxifen
These may include mild allergic reactions, such as skin rashes and hair loss (this will grow back later).
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.
Long-term side effects
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Studies have shown that postmenopausal 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 small risk is generally outweighed by the benefits of taking tamoxifen.
If detected early, treatment for endometrial cancer is usually very successful. An early warning sign is abnormal vaginal bleeding, although this is often caused by a non-cancerous condition such as polyps. If you have any abnormal vaginal bleeding, let your doctor know. Some women may have regular gynaecological check-ups.
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Tamoxifen should not be taken during pregnancy, as it may harm the developing baby. Although it can affect a woman's periods, tamoxifen is not a contraceptive. It's important to use an effective, non-hormonal form of contraception during treatment.
Women who haven't been through their menopause may still become pregnant when they've finished treatment with tamoxifen. Doctors usually advise you to wait for a few months after tamoxifen treatment finishes before you try to get pregnant. Talk to your doctor first if you’re thinking of getting pregnant.
Risk of blood clots
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.
Interaction with warfarin
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, tell your doctor straight away.
There is some research that suggests some drugs – including the antidepressants paroxetine (Seroxat®) and fluoxetine (Prozac®) – may cause tamoxifen to be less effective, but this isn't certain. Tell your doctors about any other medicines you are taking.
Bone thinning (osteoporosis)
Tamoxifen may help to prevent, and reduce the risk of, bone thinning in women who have been through their menopause.
We have more information on how you can keep your bones healthy.
Tamoxifen can lower the level of fat (lipids) in your blood. High levels of fat contribute to heart disease, but research doesn't show that tamoxifen reduces the risk of heart disease.
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. Your doctor or nurse will give you details of who to contact for advice. This should include out-of hours contact details if you need to call someone during the evening, overnight or at the weekend.
Things to remember about tamoxifen tablets
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Tamoxifen may interact with other medicines. Tell your doctor about any medicines you’re 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.
Store the tablets in the original packaging/container and at room temperature, away from moisture, direct heat and sunlight.
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.
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.
Remember to get a new prescription a few weeks before you run out of tablets and make sure you have plenty for holidays.
This section has been compiled using information from a number of reliable sources including:
British National Formulary. 63rdh edition. 2012. British Medical Association and Royal Pharmaceutical Society of Great Britain.
electronic Medicines Compendium (eMC). www.medicines.org.uk (accessed September 2012).
Kelly CM, et al. Selective serotonin reuptake inhibitors and breast cancer mortality in women receiving tamoxifen: a population based cohort study. BMJ. 2010. 340: c693.
Sweetman, et al. Martindale: The Complete Drug Reference. 37th edition. 2011. Pharmaceutical Press.
Thanks to Kavita Kantilal, E-Prescribing Pharmacist, and 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.