NSAIDs can be effective, but they are not completely benign. There are possible side effects, most commonly stomach problems. This can occur, at least in part, because most NSAIDs (such as ibuprofen and naproxen) work by blocking the action of two enzymes. The COX-2 enzyme causes pain and inflammation. The COX-1 enzyme helps protect the stomach lining. Blocking COX-1 can leave the stomach vulnerable to damaging effects of stomach acid, which may lead to an ulcers and other problems.