The short answer
HER2-positive breast cancer has too much of a specific protein that can be targeted directly. Trastuzumab, pertuzumab, and antibody-drug conjugates are drugs aimed at HER2. Ask your care team how they fit your plan.
HER2-positive means the cancer has too much of a growth protein called HER2, which is what makes it targetable.
Trastuzumab and pertuzumab block different parts of HER2 and are frequently given together with chemotherapy.
T-DM1 and trastuzumab deruxtecan are antibody-drug conjugates that carry chemotherapy directly to HER2-positive cells.
Trastuzumab can cause serious heart problems, so your doctor orders heart tests before and during treatment.
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This page is for people with HER2-positive breast cancer and those close to them. It explains what the term means, the main drugs, why your heart is checked, and what treatment is often like.
What HER2-positive actually means
HER2 is a protein on the surface of cells. NCI says some breast cancers have too much HER2. That is what your pathology report means when it says HER2-positive. Your report may also show terms such as IHC or FISH.
The important part is what follows from it. Because the cancer has too much of one specific protein, drugs can be aimed at that protein. These are called targeted therapies.
The drugs you are likely to hear about
NCI lists trastuzumab, pertuzumab, ado-trastuzumab emtansine (T-DM1), and trastuzumab deruxtecan among the targeted drugs used for breast cancer with too much HER2. NCI says targeted therapy might be started before or after surgery.
Trastuzumab is an antibody that binds HER2 and blocks its signal. Pertuzumab is another antibody that targets HER2. T-DM1 and trastuzumab deruxtecan are antibody-drug conjugates. That means an antibody is linked to a chemotherapy drug, so the drug is carried to cells that have HER2.
Which drugs you get, in what order, and for how long depends on your cancer. Your care team will explain your plan.
Heart monitoring, and why it happens
MedlinePlus says trastuzumab may cause serious or life-threatening heart problems. That is why your doctor orders tests before and during treatment to see if your heart is working well enough to receive it. Tell your team about any heart disease you have had.
Contact your care team right away if you have a cough, shortness of breath, swelling of the arms, hands, feet, ankles or lower legs, quick weight gain, dizziness, or a fast, irregular, or pounding heartbeat. Do not wait for the next scheduled test. Call 911 if you faint or have severe trouble breathing.
Statistics and outlook
Survival statistics can come from different years and different groups of people. If a number frightens you, ask your oncologist which years of data it came from and how it applies to you.
Side effects
NCI lists common side effects of targeted therapy for breast cancer, including diarrhea, liver problems, mouth sores, and hair and nail changes. Side effects differ from drug to drug. Fertility, pregnancy plans, and long-term heart health are all fair things to raise before you begin, rather than afterward.
Sources
Common questions
Why do I need heart tests during treatment?
MedlinePlus says trastuzumab may cause serious or life-threatening heart problems. Your doctor orders tests before and during treatment to see if your heart is working well enough to receive it. Tell your doctor about any heart disease first.
What heart symptoms should I report?
MedlinePlus lists cough, shortness of breath, swelling of the arms, hands, feet, ankles or lower legs, quick weight gain, dizziness, loss of consciousness, and a fast, irregular, or pounding heartbeat. Contact your care team right away if you have any of these. Call 911 for fainting or severe trouble breathing.
What is the difference between trastuzumab and trastuzumab deruxtecan?
Trastuzumab is the antibody alone, blocking the HER2 signal. Trastuzumab deruxtecan is that same antibody linked to a chemotherapy drug, so it delivers the chemotherapy to cells carrying HER2. They are used in different situations and have different side effects.
Why do the survival statistics I found online look frightening?
Statistics can come from different years and different groups of people. Ask your oncologist which years of data any number comes from and how it applies to you.
Do I still need chemotherapy if I am getting targeted therapy?
Your specific plan depends on stage, tumor size, node involvement, and hormone receptor status. Ask your care team which drugs you will get and in what order.
Questions to ask your doctor
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Written by: Cancer ExplainedSources last checked: 2026-10-16 what this meansLast updated: 2026-10-16Next planned review: 2027-07-30
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Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes, and this is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and then compared with the sources listed on it, looking for statements those sources do not back up. That check is not a sentence-by-sentence record and can miss errors. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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