Trastuzumab is used alone or with other medications to treat certain types of breast cancer. It is also used along with other medications to treat certain types of stomach cancer. The types of cancers trastuzumab is used to treat are tumors that produce more than the normal amount of a certain substance called HER2 protein.This medication is called a monoclonal antibody. It works by attaching to the HER2 cancer cells and blocking them from dividing and growing. It may also destroy the cancer cells or signal the body (immune system) to destroy the cancer cells.