A duodenal ulcer is a sore that forms on the lining of the duodenum, which is the first part of the small intestine. It occurs when stomach acid damages the protective lining, often due to factors like infection from Helicobacter pylori bacteria or the use of nonsteroidal anti-inflammatory drugs (NSAIDs). Symptoms may include abdominal pain, bloating, and indigestion. Treatment typically involves medications to reduce stomach acid and antibiotics to eliminate the H. pylori infection. Lifestyle changes, such as avoiding spicy foods and reducing stress, can also help manage symptoms and promote healing.