Tamoxifen is a hormone therapy drug to treat breast cancer in women and men. It is also sometimes called endocrine therapy. You pronounce tamoxifen as ta-mox-si-fen.

It’s a treatment for:

  • early breast cancer
  • secondary breast cancer

Women who have a high risk of breast cancer because of a family history might have tamoxifen. This is to reduce the risk of breast cancer from developing.

Early breast cancer 

You might have tamoxifen for early breast cancer:

  • to lower the risk of it coming back (recurring) after surgery
  • to shrink the cancer before surgery (neo adjuvant treatment)
  • if you are not able to have surgery

Women can have tamoxifen if they are pre menopausal or post menopausal. If you are pre menopausal you might have it on its own or with another drug to stop the ovaries from working. This is called ovarian suppression.  

Secondary breast cancer 

People with secondary breast cancer may have tamoxifen to control the breast cancer. Secondary breast cancer means the breast cancer has spread to another part of the body, for example, the liver or bones.

Tamoxifen for men

Tamoxifen is used to treat breast cancer in men. Breast cancer is rare in men so less is known about the side effects of this drug for men. Much of the research is on women taking tamoxifen. The evidence suggests the side effects for men are similar to those listed on this page.

How does tamoxifen work?

To treat breast cancer 
Many breast cancers are stimulated to grow by the sex hormones oestrogen and progesterone. These breast cancers are called hormone sensitive or hormone receptor positive breast cancers. These hormones are known more as a female hormones. But men also have a small amount of oestrogen and progesterone in their body. This means that men can also develop hormone positive breast cancer.  

Tamoxifen works by blocking the oestrogen receptors. It stops oestrogen from telling the cancer cells to grow. 

To prevent breast cancer 
As tamoxifen blocks the effects of oestrogen, the aim is to stop breast cancers from developing.

How do you have tamoxifen?

Tamoxifen comes as a tablet that you swallow. It is also available as a liquid. 

You must take tablets according to the instructions your doctor or pharmacist gives you.

You should take the right dose, not more or less.

Talk to your healthcare team before you stop taking a cancer drug, or if you miss a dose.

How often do you have tamoxifen?

You take tamoxifen daily at the same time every day. 

To treat early breast cancer, most people take tamoxifen for 5 years. In some situations, you might take it longer. This might be up to 10 years. 

If you have tamoxifen for secondary breast cancer, you take it for as long as it works, and the side effects aren’t too bad.

People who take tamoxifen to reduce the risk of breast cancer usually have it for 5 years. 


You have blood tests during your treatment. They check your levels of blood cells and other substances in the blood. They also check how well your liver and kidneys are working. You might have blood tests to check your hormone levels.

You may not need regular blood tests if you are feeling well, and the side effects are not too bad. Your doctor will let you know how often you need them.

What are the side effects of tamoxifen?

How often and how severe the side effects are can vary from person to person. They also depend on what other treatments you're having. 

When to contact your team

Your doctor, nurse, or pharmacist will go through the possible side effects. They will check how you are at your appointments. Contact your advice line as soon as possible if:

  • you have severe side effects
  • your side effects aren’t getting any better
  • your side effects are getting worse

We haven't listed all the side effects here. Remember it is very unlikely that you will have all of these side effects, but you might have some of them at the same time.

Common side effects

These side effects happen in more than 10 in 100 people (more than 10%). You might have one or more of them. They include:

Hot flushes and sweats 

We have some tips for coping with hot flushes in women and hot flushes in men. This information also includes some of the possible treatments. Talk to your doctor if your hot flushes are hard to cope with. They might be able to prescribe you some medicines.

Fluid build up 

You may have swelling of your hands and legs due to a build up of fluid (oedema). You may notice swelling in other parts of your body, for example, your eyes or lips but this is rare.

Feeling or being sick

You may feel sick when you first start taking tamoxifen, but this should improve the longer you take it. Taking tamoxifen with food or in the evening may help.

Tiredness and weakness (fatigue)

Tiredness and weakness (fatigue) can happen during and after treatment. Doing gentle exercises each day can keep your energy up. Don't push yourself, rest when you start to feel tired and ask others for help.

Vaginal changes

This might include bleeding or discharge from the vagina. Talk to your doctor or nurse if you have any bleeding or discharge. Rarely you might have polyps in the vagina. Polyps are usually non cancerous growths.

Skin rash

Skin problems include a skin rash, dry skin, and itching. This usually goes back to normal when your treatment finishes.

Rarely you might have a rash that doesn’t get better. You may notice pink or dark red patches that may develop into blisters. Or large areas of your skin peeling.

Speak to your nurse or doctor if you see any changes in your skin.


Tell your doctor or nurse if you’re feeling depressed. They can arrange for you to talk to someone and give treatment if necessary.

Occasional side effects

These side effects happen in between 1 and 10 out of every 100 people (between 1 and 10%). You might have one or more of them. They include

  • leg cramps
  • hair thinning or hair loss
  • feeling light headed
  • eye problems, such as blurred vision, due to damage of the retina or cataracts. Less commonly you might have pain or swelling in and around your eyes or lose your vision
  • an allergic reaction. Symptoms can include sudden breathing difficulties, dizziness, swelling of the mouth, face, or throat, and a skin rash. Contact your advice line if you have any of these symptoms
  • headaches
  • diarrhoea
  • itchiness on or around the vulval area – talk to your doctor as this could be an infection and might need treatment
  • changes in your taste
  • tingling and prickling sensation usually in the hands and feet
  • muscle aches
  • low levels of red blood cells (anaemia). Symptoms include shortness of breath, tiredness, and lack of energy
  • pain where the cancer is (if your cancer has not been removed)
  • high levels of a type of fat in the blood (triglyceride). High levels can cause fat build up in your bloodstream or in your organs such as your pancreas. You have blood tests to check the levels
  • blood clots that can be life threatening; signs are pain, redness, and swelling where the clot is. Feeling breathless can be a sign of a blood clot in the lung. Contact your advice line straight away if you have any of these symptoms. The risk may increase if you are having chemotherapy or other anti cancer drugs with tamoxifen. There is also a risk of having a stroke Open a glossary item with tamoxifen
  • changes in your liver enzyme levels (which can be seen in blood tests) or fatty deposits in the liver. Other less common liver problems might cause symptoms such as yellowing of the skin or eyes, pale looking poo, dark urine, or pain in your tummy (abdomen)
  • changes to the lining of the womb Open a glossary item and non cancerous growths in the womb. Rarely tamoxifen might cause womb cancer. Tell your doctor if you have unexpected or irregular vaginal bleeding
  • tamoxifen may cause endometriosis. This is when tissue similar to the lining of the womb starts to grow in other places, for example, the ovaries. This is not cancerous but can be very painful

​​​​​​​Rare side effects

These side effects happen in fewer than 1 in 100 people (less than 1%). You might have one or more of them. They include:

  • inflammation of the lungs causing breathlessness and a dry cough
  • low level of platelets Open a glossary item
  • high calcium levels in people with secondary breast cancer
  • symptoms of your breast cancer becoming worse, such as the cancer looking or feeling bigger. This is usually when you start your treatment (tumour flare)
  • inflammation of the blood vessels. You may see red or purple spots on your skin 
  • changes in your periods if you are pre menopausal
  • swelling in the ovaries. Symptoms include pain during sexual activity, bloated tummy, and needing to go for a wee more often
  • if you had radiotherapy before taking tamoxifen you may get a flare up of the skin around the radiotherapy treatment area
  • severe infection due to low white blood cells Open a glossary item. Symptoms include a fever, aching muscles, headaches, feeling cold and shivery, and generally feeling unwell. You might have other symptoms depending on where the infection is

Infections can sometimes be life threatening. You should contact your advice line urgently if you think you have an infection.

Coping with side effects

We have more information about side effects and tips on how to cope with them.

What else do I need to know?

Other medicines, foods and drink 

Cancer drugs can interact with medicines, herbal products, and some food and drinks. We are unable to list all the possible interactions that may happen. An example is grapefruit or grapefruit juice which can increase the side effects of certain drugs.

Tell your healthcare team about any medicines you are taking. This includes vitamins, herbal supplements and over the counter remedies. Also let them know about any other medical conditions or allergies you may have.

Pregnancy and contraception

This drug may harm a baby developing in the womb. It is important not to become pregnant or father a child while you are having treatment with this drug and for at least 9 months afterwards.

Talk to your doctor or nurse about effective contraception before starting treatment. Let them know straight away if you or your partner falls pregnant while having treatment.


It is not known whether this drug comes through into the breast milk. Doctors usually advise that you don’t breastfeed during this treatment.

Treatment for other conditions

If you are having tests or treatment for anything else, always mention your cancer treatment. For example, if you are visiting your dentist.

More information about this treatment

For further information about this treatment and possible side effects go to the electronic Medicines Compendium (eMC) website. You can find the patient information leaflet on this website.

You can report any side effect you have to the Medicines Health and Regulatory Authority (MHRA) as part of their Yellow Card Scheme.

Related links