An ulcer is a painful open sore that forms inside your body or on your skin. The most common type develops in the lining of your stomach or the upper part of your small intestine. These stomach ulcers feel like a burning pain in your belly, especially when you're hungry or at night.

Think of an ulcer like a raw spot that won't heal. Just as a scrape on your knee stings when you touch it, an ulcer inside your stomach burns when stomach acid touches it. The acid that normally helps digest your food irritates the damaged area, making it hurt more.

Ulcers can also form in your mouth (called canker sores), on your legs (especially in older people with circulation problems), or anywhere the protective layer of tissue gets damaged and can't heal properly. Doctors used to think stress and spicy foods caused stomach ulcers, but they discovered that most are actually caused by a specific bacterium or by taking too much pain medicine like aspirin. With proper treatment, most ulcers heal completely within a few weeks.