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Learn all about tucatinib (Tukysa), a targeted therapy for breast cancer. Find out what it is, how it’s given, and whether you might be offered it.

1. What is tucatinib?

Tucatinib is a targeted (biological) therapy. Targeted therapies interfere with processes in cells that help cancer grow.

Tucatinib is one of a group of cancer drugs called tyrosine kinase inhibitors (TKIs).

Tucatinib is the drug’s non-branded name. Its brand name is Tukysa.

2. Who might be offered tucatinib?

You may be offered tucatinib if you have HER2 positive breast cancer that has spread to another part of the body or cannot be removed by surgery.

It’s given to people who have already had two or more other treatments specifically for HER2 positive breast cancer.

3. How tucatinib works

Some breast cancer cells have a higher than normal level of a protein called HER2 on their surface, which stimulates them to grow.

Tucatinib helps control how cells grow and divide. It blocks the HER2 protein in the cancer cells, which stops the cells from growing and spreading.

4. How tucatinib is given

Tucatinib is taken by mouth as a tablet twice a day, every day. It can be taken with or without food.

Tucatinib is taken in combination with trastuzumab (Herceptin) and capecitabine.

On the days you take capecitabine, you can take it at the same time as tucatinib.

If you can’t take your tucatinib for any reason or if you are sick after taking your tablets, do not take an extra dose. Contact your treatment team as soon as possible for advice.

You’ll have tucatinib for as long as your treatment team feels you’re benefitting from the drug and any side effects are manageable.

While you are taking tucatinib you’ll have regular blood tests to check the levels of your blood cells and how well your liver and kidneys are working.

5. Side effects of tucatinib

Like any treatment, tucatinib can cause side effects. Everyone reacts differently to drugs and some people have more side effects than others. These side effects can usually be managed and those described here will not affect everyone.

Tucatinib is given together with capecitabine and trastuzumab, so you may have side effects from those treatments too.

You should be given a 24-hour contact number or told who to contact if you feel unwell or you are concerned about side effects at any time during your treatment, including at night or at the weekend.

Common side effects

Effects on the blood

Tucatinib taken with capecitabine and trastuzumab can temporarily affect the number of blood cells in the body.

You’ll have regular blood tests to check your blood count. Blood is made up of red cells, white cells and platelets. 

If the number of blood cells is too low, your next cycle of treatment may be delayed or the capecitabine dose may be reduced. 

Doctors often tailor the dose of a drug by reducing it to suit an individual and to help manage its side effects. For some people this can help them stay on a treatment for longer. 

Risk of infection

Not having enough white blood cells (neutropenia) can increase the risk of getting an infection.

Signs of an infection

Your treatment team may give you guidelines to follow for reporting signs of an infection. But generally you should contact your hospital immediately if you experience any of the following:  

  • A high temperature (over 37.5°C) or low temperature (under 36°C), or whatever your treatment team has advised  
  • Suddenly feeling unwell, even with a normal temperature  
  • Symptoms of an infection, for example a sore throat, a cough, a need to pass urine frequently or feeling cold or shivery 

Before you start tucatinib your treatment team should give you a 24-hour contact number or tell you where to get emergency care.


Having too few red blood cells is called anaemia. If you feel particularly tired, breathless or dizzy, let your treatment team know.

Bruising and bleeding

This treatment can reduce the number of platelets, which help the blood to clot.

You may bruise more easily, have nosebleeds or your gums may bleed when you brush your teeth.

Tell your treatment team if you have any of these symptoms.


You may have diarrhoea. Your treatment team or GP can prescribe medicine to help control it. 

Drink plenty of fluids to avoid becoming dehydrated.

Contact your treatment team if you have four or more episodes of diarrhoea within a 24-hour period.

Skin reactions

Your skin may become sore and red, usually on the palms of your hands and soles of your feet. This is called hand-foot syndrome or Palmar-plantar syndrome

Your treatment team may prescribe creams to help with this.

Nausea and vomiting

You may feel sick (nausea) and be sick (vomit).

You’ll be prescribed anti-sickness drugs to take home to reduce nausea or stop it happening.

If you continue to feel sick or vomit, tell your treatment team as they may be able to change your anti-sickness drugs.

Try to eat small regular meals if possible and have regular drinks.

Cancer-related fatigue (extreme tiredness)

Cancer-related fatigue is extreme tiredness that doesn’t go away with rest or sleep.

It’s a very common side effect of breast cancer treatment and may affect you physically and emotionally.

It has many causes, from psychological factors such as the stress of coping with the diagnosis, to physical ones such as the side effects of treatment or progression (growth and spread) of the cancer.

If you think you have fatigue, tell your GP or treatment team so you can be fully assessed and offered advice on how to manage your energy levels.

Find out more about managing fatigue.

Sore mouth 

Your mouth and gums can become sore and small ulcers may develop.

Looking after your mouth, including your teeth and gums, is very important during treatment. 

Your treatment team will advise you about suitable mouthwashes or medicine if these problems occur. 

If you need to have any dental treatment, talk with your oncologist about the best time to have this.

Loss of appetite

You may not feel like eating, especially if you’re feeling sick. Your sense of taste may also change, and some food and drink may taste different. 

Eating small, frequent meals or snacks can help keep up your food intake. If you struggle to maintain a healthy weight, ask your GP or specialist about being referred to a dietitian. 

Liver problems

Tucatinib can affect how the liver works. Tell your treatment team if you notice any itching or yellowing of the eyes or skin (jaundice).

You will have regular blood tests to check your liver function while taking tucatinib.

Abdominal pain

You may experience abdominal pain and discomfort or feel bloated.

Let your treatment team or GP know. They may prescribe medication to help.

Other common side effects

Other common side effects of tucatinib include:

  • Headaches
  • Skin rashes
  • Joint pain
  • Weight loss
  • Nose bleeds

Your treatment team can advise you on how to manage these.

Less Common side effects

Seizures (fits)

Breast cancer that has spread to the brain can cause seizures (fits).

People receiving tucatinib for secondary breast cancer in the brain may experience seizures.

Seizures can affect people in different ways. Some seizures cause loss of consciousness and the body may jerk and twitch. Others happen without much movement, causing problems like loss of awareness, unusual sensations or an inability to talk.

Seizures typically pass in a few seconds or minutes. They can happen without warning and can be very frightening for you and people around you.

Tell your treatment team straight away if you experience a seizure.

6. Other important information

Allergic reaction

Very occasionally allergic reactions to a drug can occur. Reactions can vary from mild to severe, although severe reactions are uncommon. 

If you have an allergic reaction to tucatinib, it’s more likely to happen the first time you have the treatment.

Symptoms include:

  • Flushing
  • Skin rash
  • Itching
  • Back pain
  • Shortness of breath
  • Feeling faint
  • Fever or chills

If you experience any of these symptoms, let your treatment team or chemotherapy nurse know immediately. 

Blood clots  

People with breast cancer have a higher risk of blood clots such as a deep vein thrombosis (DVT). Their risk is higher because of the cancer itself and some treatments for breast cancer. If the cancer has spread to other parts of the body (secondary breast cancer), this also increases the risk.

People with a DVT are at risk of developing a pulmonary embolism (PE). This is when part of the blood clot breaks away and travels to the lung. 

Blood clots can be harmful but are treatable so it’s important to report symptoms as soon as possible.

Blood clot symptoms

If you have any of the following symptoms, contact your treatment team or go to your local A&E department immediately:

  • Pain, redness/discolouration, heat and swelling of the arm or leg
  • Swelling, redness or tenderness where a central line is inserted to give chemotherapy - for example in the arm, chest area or up into the neck
  • Shortness of breath
  • Pain or tightness in the chest
  • Unexplained cough or coughing up blood

Find out more about blood clots.

Medicines to avoid when taking tucatinib

There are a number of drugs that should not be taken with tucatinib, so it’s important to tell your treatment team about any prescribed or over-the-counter medicines you are taking.

Many people consider taking herbal medicines or supplements while having treatment for breast cancer.

Tucatinib must not be taken with anything containing St John’s wort. You should ask your treatment team before taking any herbal medicines or supplements.

Sex, contraception and pregnancy

It’s important you don’t get pregnant when you’re taking tucatinib or for at least one week after the last dose because the drug can harm a developing baby.

If you haven’t been through the menopause, talk to your treatment team about the most suitable method of contraception for you. 

Women should use an effective barrier method of contraception such as condoms during treatment and for at least one week after the last dose.

Men taking tucatinib who have a sexual partner who can become pregnant should use a reliable method of contraception while taking tucatinib and for at least one week after the last dose.

You can still have sex during treatment. It’s not known if tucatinib can pass into vaginal fluids or semen but this can’t be completely ruled out as some drugs can pass into the blood and some other body fluids. Most hospital specialists will advise using barrier methods of contraception, such as condoms, during treatment and for a few days after treatment is given.


The impact of tucatinib on fertility is not currently known. It’s important to discuss any fertility concerns with your treatment team before you begin your treatment.


Breastfeeding is not recommended while having tucatinib, or within a week of the last dose. This is because there’s a risk the drugs could be passed on through breast milk.

Travel and vaccinations

If you’re planning a holiday or need to travel overseas, check with your treatment team first. 

You shouldn’t have any live vaccines while you’re having treatment. Live vaccines include mumps, measles, rubella (German measles), polio, BCG (tuberculosis), shingles and yellow fever. 

Live vaccines contain a small amount of live virus or bacteria. If you have a weakened immune system, which you may do during treatment, they could be harmful.

It’s safe to have these vaccinations six months after your treatment finishes. Talk to your GP or treatment team before having any vaccinations.

If anyone you have close contact with needs to have a live vaccine speak to your treatment team or GP. They can advise what precautions you may need to take depending on the vaccination.

Flu vaccination 

Anyone at risk of a weakened immune system, and therefore more prone to infection, should have the flu vaccine. This includes people due to have, or already having, chemotherapy. 

The flu vaccine is not a live vaccine so doesn’t contain any active viruses. Talk to your chemotherapy team or breast care nurse about the best time to have your flu jab.

Coronavirus (Covid-19) vaccination  

People having tucatinib are advised to speak to their treatment team before having a coronavirus (Covid-19) vaccination.

7. Further support

Everyone’s experience of being diagnosed with secondary breast cancer is different, and people cope in their own way.

For many people, uncertainty can be the hardest part of living with secondary breast cancer.

You may find it helpful to talk to someone else who’s had a diagnosis of secondary breast cancer.

Chat to other people living with secondary breast cancer on our online forum

Meet other people with a secondary diagnosis and get information and support at a Living with Secondary Breast Cancer meet-up

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Quality assurance

Last reviewed in January 2022. The next planned review begins in April 2023.

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