Complex sentence: Dyspepsia is a common problem, and is frequently due to gastroesophageal reflux disease (GERD) or gastritis, but in a small minority may be the first symptom of peptic ulcer disease (an ulcer of the stomach or duodenum) and occasionally cancer.
Split into simpler sentences: Dyspepsia is a common problem, and is frequently associated with, gastroesophageal reflux disease (GERD) or gastritis. In a small minority it may be the first symptom of peptic ulcer disease (an ulcer of the stomach or duodenum) and occasionally cancer.