Aspirin. Molecular model of an aspirin molecule. Atoms are represented as balls and are colour coded: carbon (grey), oxygen (red) and hydrogen (white). Aspirin (acetylsalicylic acid, C9H8O4) is a non-steroidal anti-inflammatory drug (NSAID). It is taken for its painkilling (analgesic) properties, and it also helps to reduce fevers. It can also reduce blood clotting, and is used to treat heart attack patients and those at risk from a stroke. Side-effects include erosion of the stomach lining, which may lead to gastric ulcers.