Perjeta is a monoclonal antibody used in the treatment of HER2-positive breast cancer. It works by preventing the HER2 receptor from interacting with other receptors. It belongs to the class of drugs known as HER2 inhibitors and is FDA approved to be used in combination with Herceptin and chemotherapy. Perjeta has been shown to significantly improve progression-free survival in patients with metastatic HER2-positive breast cancer. It is administered intravenously once every three weeks and can be continued up to a year or more in certain patients who are responding well to treatment. Common side effects of Perjeta include fatigue, nausea, diarrhea, and hair loss.