How do pain relievers cause GI bleeding and ulcers?

Some pain relievers, particularly nonsteroidal anti-inflammatory drugs (NSAIDs) like aspirin and ibuprofen, work by inhibiting prostaglandin production. Prostaglandins are chemicals that protect your digestive tract from acid. Prostaglandins also are responsible for pain, fever and inflammation. Because NSAIDs decrease the amount of prostaglandins that protect areas of your digestive tract, overuse can lead to ulcers and GI bleeding. This is why your doctor might hesitate to recommend NSAIDs for long-term use.