How Does Trastuzumab Work? — The Anti-HER2 Mechanism Explained

HER2-positive means the laboratory found an unusually high number of HER2 proteins on the surface of your cancer cells. In a normal cell, HER2 is present in small amounts. In HER2-positive cancer, the overabundance of this protein keeps growth signals permanently switched on. This finding comes from testing your tumour tissue — usually through a staining technique called IHC, sometimes confirmed by a complementary test called FISH or ISH — and it is what makes trastuzumab a relevant treatment option.

No. Trastuzumab is a targeted therapy — a monoclonal antibody designed to attach to one specific protein, HER2. Chemotherapy disrupts cell division across the body, affecting both cancer cells and normal rapidly dividing cells. Trastuzumab is more selective. Its side-effect profile is different from chemotherapy, and it does not typically cause hair loss or severe nausea. Some people receive both as part of their treatment plan, but they work by different mechanisms and are not the same type of treatment.

The most common side effects during infusion are reactions such as chills, mild fever, or breathlessness — your team monitors you closely for this, especially during the first infusion. Over time, the main concern is the heart: trastuzumab can reduce the heart's pumping efficiency in a proportion of patients, which is why echocardiograms are done before and during treatment. Many people tolerate trastuzumab well. Tell your team straight away if you notice breathlessness, swollen ankles, or a fast or irregular heartbeat.

Response is assessed through imaging — scans at planned intervals during treatment — and, in some situations, blood markers. In early-stage breast cancer, oncologists also look at what surgery finds in the removed tissue: the absence of cancer cells in the surgical specimen, called a pathological complete response, is a marker used in this setting. Your oncologist will explain what they are measuring and when the first assessment will take place.

Yes. Trastuzumab was first established in breast cancer, but it is also used for HER2-positive gastric cancer and cancer of the gastro-oesophageal junction, where ASCO and ESMO guidelines include it in standard first-line regimens. Research into its role in other HER2-expressing cancers, including certain colorectal and lung cancers, is ongoing. Eligibility always requires HER2 testing of your specific tumour, regardless of where the cancer originated.

Trastuzumab alone does not typically cause hair loss. Hair loss is associated with certain chemotherapy drugs, not with monoclonal antibodies like trastuzumab. If your treatment plan includes both trastuzumab and a chemotherapy drug that causes hair loss — which is common in some breast cancer regimens — the hair loss comes from the chemotherapy part of the plan. Ask your oncologist which drugs in your specific regimen are responsible for which side effects, so you know what to prepare for.