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Trastuzumab emtansine (Kadcyla)

Learn how certain types of breast cancer can be treated with trastuzumab emtansine, also known by its brand name, Kadcyla. Find out who it’s for and the possible side effects.

1. What is trastuzumab emtansine?

Trastuzumab emtansine is a targeted therapy.

Targeted therapy is the name given to a group of drugs that block the growth and spread of cancer. They target and interfere with processes in the cells that help cancer grow.  

Trastuzumab emtansine is the drug’s non-branded name. You may hear it called Kadcyla, its brand name, or TDM1, its chemical name.

Trastuzumab emtansine is a combination of 2 drugs:

  • Trastuzumab (Herceptin), which is a targeted therapy drug
  • Emtansine, which is a chemotherapy drug

2. Who might be offered trastuzumab emtansine?

Trastuzumab emtansine is used to treat breast cancer that is HER2-positive.

You might be offered trastuzumab emtansine if:

  • You’ve had surgery for primary breast cancer and some cancer is still in the breast or lymph nodes after neoadjuvant (before surgery) treatment with docetaxel or paclitaxel chemotherapy and a HER2-targeted therapy (trastuzumab or pertuzumab)
  • You have secondary breast cancer or that cannot be removed by surgery and have already had trastuzumab and chemotherapy drugs (docetaxel or paclitaxel), either together or separately

3. How does trastuzumab emtansine work?

Some breast cancer cells have a higher-than-normal level of a protein called HER2 on their surface, which makes them grow more quickly. This is known as HER2-positive breast cancer. 

Trastuzumab attaches to the HER2 proteins and can stop cancer cells from growing. When the trastuzumab attaches to the proteins, it delivers emtansine directly into the breast cancer cells. The emtansine can destroy the cancer cells with not much damage to other healthy cells. This means there are usually fewer side effects than you might have with other drugs.

Trastuzumab also helps the body’s immune system to destroy cancer cells.

There are different tests to measure HER2 levels, which are done on breast tissue removed during a or surgery. Trastuzumab emtansine cannot be used to treat HER2-negative cancers.

4. How is trastuzumab emtansine given?

Trastuzumab emtansine is given into a vein (intravenously). This will usually be as an infusion (drip) either in the back of the hand or lower arm.

There are other ways to have medication intravenously too. Your treatment team will discuss which is best for you based on how easy it is to access suitable veins and what you prefer.

You will have the first infusion over 90 minutes. You’ll then be monitored for at least another 90 minutes to make sure you don’t have a bad reaction to the treatment.

If you do not have a bad reaction to your first infusion, your future infusions will usually be given over 30 minutes and you’ll be monitored for at least another 30 minutes.

After surgery in primary breast cancer

You’ll have trastuzumab emtansine as an outpatient every 3 weeks for up to 14 cycles.

For secondary breast cancer or locally advanced breast cancer

You’ll have trastuzumab emtansine as an outpatient every 3 weeks for as long as your treatment team feels you’re benefitting from the drug and any side effects are manageable.

5. Side effects

Common side effects of trastuzumab emtansine

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

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. This includes at night or at the weekend.

Effects on the blood

Trastuzumab emtansine can temporarily affect the number of blood cells in the body. Blood is made up of red cells, white cells and platelets.

You’ll have regular blood tests. If these show the number of blood cells is too low, your next cycle of treatment may be delayed or the dose reduced.

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

Risk of infection

Not having enough white blood cells 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 starting treatment, you should be given a 24-hour contact number or told where to get emergency care by your treatment team.

You may need treatment for an infection.

Sometimes your doctor may recommend injections of drugs called growth factors to stimulate the production of white blood cells to reduce your risk of infection.


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

Trastuzumab emtansine can reduce the number of platelets, which help the blood to clot.

You may also 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.

Extreme tiredness (cancer related fatigue)

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

It’s a very common side effect of 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 growth and spread of the cancer. 

Fatigue affect how you cope with your cancer and its treatment. It can also make everyday activities harder and affect your quality of life. 

If you think you have fatigue, tell your treatment team or GP. They can assess you and give you advice on how to manage your energy levels.

Sore mouth

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

You’ll be given mouthwash to try to reduce soreness and stop mouth ulcers developing.

See your dentist for a check-up before you start your treatment and avoid dental work during treatment if possible.

If you do need to have dental work during treatment, talk with your treatment team about the best time to have it done.

Numbness and tingling in hands or feet

Trastuzumab emtansine can affect the nerves in your hands and feet, causing numbness or tingling (peripheral neuropathy). In most cases it’s mild and usually goes away soon after treatment stops. If it’s severe, your treatment team might need to reduce the dose, delay treatment or stop it completely.

If you have numbness or tingling, tell your treatment team so they can monitor it.

Difficulty sleeping

For some people, trastuzumab emtansine can affect sleep. If you have difficulty sleeping (insomnia), some simple things like limiting caffeine in the afternoon and evening, keeping your room dark and quiet, and going to bed and getting up at the same time each day may help.

Relaxation exercises can also be helpful. There are apps, podcasts and CDs that can guide you through these techniques.

If your insomnia carries on, your GP may prescribe something to help you sleep.

Feeling sick (nausea) and being sick (vomiting)

You may have nausea and vomiting, but most people will not actually be sick.

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.

Diarrhoea or constipation

You may have diarrhoea or constipation. Your treatment team or GP can prescribe medicine to help control these side effects.

Contact your specialist chemotherapy team or your 24-hour contact number if you have diarrhoea 4 or more times in 24 hours.

Low levels of potassium in the blood

You’ll have regular blood tests before and during treatment to check your potassium levels.

If your potassium levels are low your treatment team may prescribe supplements.

Potassium levels usually go back to normal when your treatment finishes.

Other common side effects

Other common side effects of trastuzumab emtansine include:

  • Headaches
  • Shortness of breath and coughing
  • Dry mouth
  • Tummy (abdominal) pain
  • Muscle or joint pain

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

Less common side effects of trastuzumab emtansine

Allergic reaction

If you have an allergic reaction to trastuzumab emtansine, it’s more likely to happen the first time you have the treatment. This is why your first treatment is given over 90 minutes.

Before your treatment starts, you’ll be given drugs to reduce the risk of an allergic reaction.

You’ll be monitored closely during your treatment so that any reaction can be dealt with immediately. Symptoms include:

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

Heart changes

Trastuzumab emtansine may cause changes to your heart by weakening the heart muscle. This is usually temporary, but for a small number of people it may be permanent.

Before you start treatment, you’ll have tests to make sure your heart is working normally. This could be an echocardiogram (echo) or a multiple-gated acquisition (MUGA) test.

You should continue to have tests (usually an echocardiogram) every 3 to 6 months depending on your situation, and every 6 months for 2 years after your last dose of trastuzumab emtansine.

Contact your treatment team if you develop any breathlessness, chest pain, changes to your heartbeat or swollen ankles.

Skin changes

You may get a rash and your skin may be itchy. Let your treatment team know if you have any skin changes as they can prescribe creams or tablets to help.

Hand-foot syndrome (palmar-plantar erythrodysesthesia)

You may develop soreness, redness and peeling on the palms of your hands and soles of your feet. This is known as hand-foot syndrome (palmar-plantar erythrodysesthesia), and may cause tingling, numbness, pain and dryness.

Keeping the feet and hands clean, dry and well-moisturised can help.

If you have skin reactions, mention this to your treatment team so the symptoms can be managed.

Hair thinning

You may notice your hair thins while having your treatment. However, it’s unusual to lose your hair.

Dizziness and memory problems

Trastuzumab emtansine can make you feel dizzy or become forgetful. Let your treatment team know if you have these symptoms.

Other less common side effects

  • Eye problems (such as dry, sore or watery eyes)
  • Raised blood pressure
  • Indigestion
  • Taste changes

Rare side effects of trastuzumab emtansine

Pain in the injection site 

Trastuzumab emtansine may leak outside of the vein while it’s being given, which is called extravasation.

Tell the nurse giving the chemotherapy immediately if you have pain, stinging or a burning feeling around the cannula (small plastic tube) while the drug is being given.

Liver changes

Trastuzumab emtansine can affect how the liver works. You’ll have blood tests to check your liver function while you’re having treatment.

Sometimes, treatment may need to be delayed or the dose reduced if the blood tests show any problems with your liver.

Problems breathing (pneumonitis)

Trastuzumab emtansine can cause breathing problems, called pneumonitis. You may also hear it called interstitial lung disease (ILD).

Contact your treatment team if you have shortness of breath and coughing.

6. Other important information

Blood clots

People with breast cancer have a higher risk of blood clots such as a DVT (deep vein thrombosis).

If you have a DVT, there’s a risk part of the blood clot could break away and travel to the lung. This is known as a pulmonary embolism (PE).

Blood clots can be life-threatening and should be treated quickly.

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.

Sex, contraception and pregnancy

You can still have sex during treatment. It’s thought chemotherapy drugs cannot pass into vaginal fluids or semen, but this cannot be completely ruled out.

Most treatment teams will advise using barrier methods of contraception, such as condoms, during treatment and for a few days after chemotherapy is given. 

You’re advised not to become pregnant while having treatment because trastuzumab emtansine can harm a developing baby. If you have not been through the menopause, talk to your team about the most suitable method of contraception for you. It’s still possible to become pregnant even if your periods are irregular or have stopped.


At the moment, doctors don’t know whether trastuzumab emtansine affects fertility.

Conversations about fertility can be difficult, but it’s still important to discuss any fertility concerns with your treatment team before you begin your treatment.

You can always call our helpline to discuss any concerns with one of our nurses, or you can read our information about fertility and breast cancer treatment.


You usually shouldn’t breastfeed while having trastuzumab emtansine, or within 7 months of your last dose. This is because there’s a risk the drugs could be passed on to your baby through your 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 chemotherapy. Live vaccines include:

  • Mumps
  • Measles
  • Rubella (German measles)
  • Polio
  • BCG (tuberculosis)
  • The Zostavax shingles vaccine (there is another non-live shingles vaccine called Shingrix that people with weakened immune systems can have)
  • 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 chemotherapy, they could be harmful.

It’s safe to have these vaccinations 6 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.

COVID-19 vaccination

If you’re having chemotherapy or targeted therapies, you’re advised to speak to your treatment team about the best time to have a COVID-19 vaccination.

Flu vaccination

People due to have, or already having, chemotherapy should have the flu vaccine. This is because they’re more at risk of having a weakened immune system.

The flu vaccine is not a live vaccine so doesn’t contain any active viruses.

Talk to your treatment team about the best time to have your vaccine.

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Quality assurance

Last reviewed in April 2024. The next planned review is in April 2027.

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