The first time a woman receives Herceptin, the most common side effects are fever and chills. Some women also have pain, weakness, nausea, vomiting, diarrhea, headaches, difficulty breathing or rashes. Side effects usually become milder after the first treatment.
Herceptin also may cause heart damage, leading to possible heart failure. It can also affect the lungs, which may cause serious breathing problems. Before you receive Herceptin, your doctor will check your heart and lungs, and during treatment, your doctor will watch for signs of lung problems.