Trastuzumab: Uses, Side Effects and What to Expect

Last updated

September 14, 2026

Disclaimer: This information is intended to help you understand your treatment. Always discuss your health and treatment plan with your doctor or nurse, who can answer your specific questions.

If your doctor has recommended trastuzumab, you may have questions about how it fits into your cancer treatment. Why is it being given alongside other medicines? What happens at each appointment? And why do you need a heart scan before starting?

Trastuzumab is a targeted treatment, not chemotherapy. It acts on a protein called HER2 on cancer cells. In breast cancer, it is often given with chemotherapy and, for some patients, another targeted medicine called pertuzumab.

Trastuzumab at a glance

What it does: Treats HER2-positive cancers—cancers with high amounts of a protein called HER2.
Where it is used: Mainly breast cancer and certain cancers of the stomach or where the food pipe meets the stomach.
How it is given: Through a drip into a vein or, for some breast cancer treatments, an injection under the skin.
How often: Usually every three weeks; some schedules are weekly.
Important precaution: Regular heart checks are needed because trastuzumab can affect the heart’s pumping action.

Why has trastuzumab been recommended?

HER2 is a protein on the surface of cells that helps control their growth. Some cancers have much more of this protein than usual. These are called HER2-positive cancers.

Trastuzumab attaches to HER2 and blocks signals that encourage cancer cells to grow. It also helps the body’s own defences recognise and attack these cells. HER2 testing is necessary before starting trastuzumab to confirm that the cancer is HER2-positive.

The purpose of treatment depends on your situation:

Before breast surgery (neoadjuvant therapy): Trastuzumab is given with chemotherapy to help shrink the cancer. Pertuzumab may also be included, depending on the size and extent of the cancer.
After breast surgery (adjuvant therapy): Trastuzumab is used to reduce the chance of the cancer returning. Pertuzumab may be added when the risk of recurrence is higher.
When cancer has spread: Trastuzumab may be combined with other medicines to help control its growth. For HER2-positive breast cancer, these may include pertuzumab and chemotherapy. It is also used with chemotherapy for certain HER2-positive cancers of the stomach or its junction with the food pipe.

Approval and availability vary between countries. Your cancer team can explain whether the treatment described here is approved and appropriate for your situation.

Medicines with similar names

Trastuzumab emtansine and trastuzumab deruxtecan are different medicines. Their uses and side effects are not the same as those of trastuzumab.

What happens at a treatment appointment?

There are two ways to administer trastuzumab. The preparations for these routes are different and must be used by the intended route:

A drip into a vein: The first dose is usually given over 90 minutes. Later doses may be administered faster if the first dose is well tolerated.
An injection under the skin of the thigh: A preparation used for breast cancer takes about 2–5 minutes to inject. You will still need time for checks and observation, so the whole appointment takes longer.

Your team will watch for reactions during and after treatment, particularly with the first dose. Tell the nurse if you feel unwell rather than waiting for the treatment to finish.

A separate injection combines trastuzumab with pertuzumab, another HER2-targeted medicine. This is an option when your treatment plan requires both drugs; it is not simply another form of trastuzumab alone.

What is a trastuzumab biosimilar?

A biosimilar is a version of trastuzumab shown to have comparable effectiveness and safety to the original medicine. Approved biosimilars may be less expensive; a lower price does not mean they are less effective. Your team can help you compare locally available options and costs.

How long will treatment continue?

Appointments are usually every three weeks, although some drip schedules are weekly. The total length of treatment depends on why you are receiving it:

For early breast cancer: Treatment commonly lasts about one year in total, including doses given before surgery. Your plan may change according to your response and side effects.
For cancer that has spread: Treatment may continue while it is controlling the cancer and side effects remain manageable.

If you miss an appointment, contact your treatment team promptly to arrange the next dose.

What side effects should you know about?

Your experience will depend partly on whether you receive trastuzumab alone or with other cancer medicines. A symptom that develops during treatment is not necessarily caused by trastuzumab itself.

Reactions during or shortly after a dose

Fever, chills, headache, feeling sick or a rash can occur, especially with the first treatment. Tell your nurse immediately if these develop. The team can assess the reaction and may slow or pause the drip or give medicine to help.

Other possible effects

These include:

Loose stools or diarrhoea.
Feeling sick.
Headache or muscle and joint aches.
Soreness or redness where an injection is given.

Tiredness can also occur. However, chemotherapy, the cancer itself and other health problems can contribute. Tell your team if tiredness is new, marked or affecting your daily activities, so they can assess the cause.

Let your team know about symptoms that persist or interfere with eating, drinking or usual activities. They can advise you on how to manage them.

Why are heart checks needed?

Trastuzumab can weaken the heart’s pumping action. This can sometimes happen before you notice symptoms, which is why checks are needed even when you feel well.

You will usually have 2D echocardiography (2D echo) before treatment and about every three months during it. This test checks how well your heart pumps blood. Further checks may be needed after treatment finishes. Your team will explain your schedule.

If the scan shows a significant change, your doctor may pause or stop trastuzumab and arrange treatment for the heart problem.

When should you seek medical help?

Serious allergic reactions and lung problems can also occur. The following symptoms should not wait until your next appointment.

Seek emergency care for:

Severe difficulty breathing.
Swelling of the tongue or throat.
Chest pain or fainting.

If these happen while you are receiving treatment, alert the staff immediately.

Contact your cancer team urgently for:

New or worsening breathlessness, especially when lying down.
A persistent new cough, swollen feet or ankles, sudden weight gain, or a racing heartbeat.
A temperature of 38°C or above, shaking chills, or feeling suddenly very unwell, particularly if you are also receiving chemotherapy. Follow a lower temperature threshold if your team has given you one.

Keep your treatment centre’s emergency contact number where you and your family can find it easily.

What should you discuss before starting?

Tell your team about heart or lung problems, high blood pressure, previous reactions to medicines, and all medicines or supplements you take.

Pregnancy and breastfeeding

Trastuzumab can harm an unborn baby. If you could become pregnant, use effective birth control during treatment and for seven months after the last dose.
Tell your team immediately if you think you may be pregnant during this period.
If you are breastfeeding, discuss this before starting treatment. Your team will advise you about feeding your baby safely.