Duodenitis is defined as a condition of inflammation in the lining of the duodenum, the first section of the small intestine. A. Causes 1. Medication Prolonged use of nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen (Advil, Motrin), naproxen (Aleve), or aspirin, in some people, it can lead to irritation causes of  inflammation in the lining …