I remember reading an article when COX-2 inhibitors were first invented. It seems that a molecule for pain transmission is somewhat similar to a molecule used in the production of the stomach lining. All existing NSAIDS block both molecules, which causes both pain to be blocked, but also the production of the stomach lining, leading to ulcers. COX-2 inhibitors were specifically engineered to block the pain molecule without blocking the stomach lining molecule.

Or something like that. And I have no idea what the connections to heart and/or stroke problems are.