Tamoxifen

Tamoxifen is a hormone therapy for breast cancer. It can be used both before and after the menopause. It is occasionally used to treat other types of cancer, but the information here refers to its use for breast cancer.

Many breast cancers are stimulated to grow by the female sex hormones oestrogen and progesterone. These breast cancers are called hormone sensitive or hormone receptor positive. Tamoxifen is usually prescribed for women who are ER positive. That means that their breast cancer cells have oestrogen receptors (ER). The oestrogen receptor is the part of the breast cancer cell that oestrogen locks on to, stimulating the cell to divide and grow. Tamoxifen blocks the oestrogen receptor. It lowers the risk of breast cancer coming back (recurring) after surgery or developing in the other breast. 

Breast cancer is rare in men but tamoxifen can be a helpful treatment if the cancer is ER positive. We have information about breast cancer in men, which includes information about the possible side effects of tamoxifen treatment.

To find out if your cancer has hormone receptors, your specialist will arrange tests on your breast cancer cells. Your specialist may also prescribe tamoxifen if your cancer cells are ER negative, but have progesterone receptors (are PR positive).

Tamoxifen comes as a tablet or liquid that you swallow. You take it daily. You may find it easiest to take it at the same time every day. It is important to keep these tablets out of the reach of children.

There is some research showing that tamoxifen can interact with drugs that contain cytochrome P enzymes (CYP). This includes some types of antidepressants called SSRIs, in particular paroxetine, as well as drugs to treat heart conditions, acid reflux and a few others. This means that the tamoxifen doesn’t work as well as it should and may increase the risk of the cancer coming back. We need more research to confirm this. Tell your doctor if you are taking any other medicines.

It is very important that you take tablets according to the instructions your doctor or pharmacist gave you. Whether you have a full or empty stomach, for example, can affect how much of a drug gets into your bloodstream. You should take the right dose, not more or less. And never stop taking a cancer drug without talking to your specialist first.  There has been a lot of research looking into how long women should take tamoxifen after breast cancer.  If you don't carry on taking it for the full time, you won't get the full benefit of protection against your breast cancer returning.

There is general information about hormone therapies in the cancer treatment section.

We've listed the side effects associated with tamoxifen below. You can use the links to find out more about each side effect. Or go to the cancer drug side effects section for general information.

More than 10 in every 100 people have one or more of these.

• Hot flushes and sweats – around 45% of women have moderate to severe hot flushes and sweats while taking tamoxifen
• Changes to your periods – if you haven’t had your menopause your periods may become irregular. Some women find their periods stop. They usually start again within 6 to12 months of treatment finishing. However, for some women close to the time of their natural menopause they don’t start again
• Fatigue (tiredness) affects about 1 out of 4 women (25%)
• Painful joints happen in about 1 out of 4 (25%) women
• Tamoxifen may have a harmful effect on a developing baby and before you start treatment you should let your doctor or nurse know if there is any possibility that you may be pregnant
• Tamoxifen is not a contraceptive and even if your periods have stopped, you could become pregnant while you are on treatment – discuss contraception with your doctor or nurse before you start your treatment
• Feeling or being sick affects about 1 out of 5 women (20%) – it usually happens at the start of treatment and goes after a few days or weeks, but tends to be mild and easily controlled by anti sickness tablets

Between 1 and 10 in every 100 people have one or more of these.

• Discharge from the vagina, dryness and itching affect about 1 in 10 women – tell your doctor or nurse if you have any of these side effects
• Fluid retention may cause ankle and or finger swelling (or weight gain) in about one in 10 people (10%)
• Weight gain – fewer than 1 in 10 people (10%) put on weight while they are taking tamoxifen.
• Headaches
• Low mood or depression – about 1 in 10 people (10%) treated have some change in their mood. Let your doctor or nurse know if you feel low or depressed
• Hair thinning is usually only slight and not noticeable

Fewer than 1 in 100 people have these.

• Tumour flare – if you have cancer that has spread to your bones, you may have some increased pain when you first start taking tamoxifen. This is called tumour flare. Very rarely it makes you feel sick, thirsty or constipated. These symptoms can be signs that the level of calcium in your blood has gone up. If you have any of these symptoms, tell your doctor or nurse.
• Your risk of blood clots (thrombosis) can slightly increase when you take tamoxifen – if you or a close relative have had a blood clot in the past tell your doctor. They may want to change you to a different type of hormone therapy
• Liver changes that are very mild and unlikely to cause symptoms – the liver will almost certainly go back to normal when treatment is finished, but you will have regular blood tests to check how well your liver is working
• Eye problems can very occasionally occur – if you notice any changes in your eyesight tell your doctor
• A skin rash
• Womb cancer – there is a very slight increase risk of developing a cancer of the womb. If you have any abnormal bleeding, or other symptoms that concern you, tell your doctor or nurse so that you can have a check up.

Tamoxifen can react with the drug warfarin. If you are taking warfarin you will need to have regular blood tests, and your dose of warfarin may need to be adjusted.

The side effects above may be mild or more severe. A side effect may get better or worse through your course of treatment, or more side effects may develop as the course goes on. This depends on

• How many times you've had the drug before
• Your general health
• The amount of the drug you have (the dose)
• Other drugs you are having

Talk to your doctor, pharmacist or nurse about all your side effects so that they can help you manage them. You should have a contact number for your chemotherapy nurse, clinic or ward nurse. You can ring if you have any questions or problems. They can give you advice or reassure you. If in doubt, call them.

Tell your doctor about any other medicines you are taking, including vitamins, herbal supplements and other over the counter remedies – some drugs can react together.