A gastrinoma is a rare type of tumor that usually forms in the pancreas or the small intestine. It produces excessive amounts of the hormone gastrin, which stimulates the stomach to produce more acid. This can lead to severe stomach ulcers and other digestive issues, causing symptoms like abdominal pain, diarrhea, and heartburn. Most gastrinomas are part of a condition called Zollinger-Ellison syndrome, where multiple tumors can occur. Diagnosis often involves blood tests to measure gastrin levels and imaging studies to locate the tumor. Treatment may include medications to reduce acid production and surgery to remove the tumor if possible.