Erlotinib is an epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor primarily used as an active pharmaceutical ingredient (API) in the manufacture of medications for non-small cell lung cancer (NSCLC), especially in patients with specific EGFR mutations, and pancreatic cancer. By blocking EGFR, which is often overactive in these cancers, erlotinib helps slow or stop cancer cell growth. This powder-form API (typically ≥97.5% purity, white to off-white to gray) is used in pharmaceutical research and production. The formulated drug is administered orally and may cause side effects such as skin rash, diarrhea, and fatigue.