Ulcers are open sores on any part of your body, inside or outside. Most commonly, ulcers are found in the stomach, and they are usually caused by a bacterium called H. pylori, which can inflame the stomach lining. Ulcers can also be caused by nonsteroidal antiinflammatory drugs (NSAIDs) or Zollinger-Ellison syndrome. By maintaining a healthy lifestyle, you can help prevent your stomach from forming an ulcer.