Many anti-inflamtory drugs reduce the production of prostaglandins, chemicals that do all sorts of things. They cause the swelling and pain from inflamed tissue, which is actually a good thing, as it tends to make you protect the injury. They play a role in causing you to have a fever. They support the function of platelets in blood clotting, which is why you bleed more if you are on these anti-inflamatory drugs, and they stimulate production of the mucus layer that protects your stomach from digestive acids. Decreasing that can cause stomach problems.