Erlotinib is primarily used in the treatment of non-small cell lung cancer (NSCLC) and pancreatic cancer. It works by inhibiting the epidermal growth factor receptor (EGFR), which is often overactive in these types of cancers. By blocking EGFR, erlotinib helps to slow down or stop the growth of cancer cells. It is typically prescribed for patients with advanced or metastatic cancer, especially those whose tumors have specific EGFR mutations. The drug is taken orally in the form of tablets and is often used when other treatments, such as chemotherapy, have not been effective. Common side effects include skin rash, diarrhea, and fatigue, but it has shown significant benefits in extending survival and improving quality of life for many patients.