Erlotinib mesylate is primarily used in the treatment of non-small cell lung cancer (NSCLC) and pancreatic cancer. It functions as a tyrosine kinase inhibitor, targeting the epidermal growth factor receptor (EGFR) to inhibit the growth and spread of cancer cells. In NSCLC, it is often prescribed for patients with specific EGFR mutations, either as a first-line treatment or after other therapies have failed. For pancreatic cancer, it is typically used in combination with gemcitabine to improve survival rates in advanced stages. The drug is administered orally, allowing for convenient outpatient treatment. However, its use requires careful monitoring for side effects such as skin rash, diarrhea, and liver function abnormalities.