Analgesic drug molecules. Molecular models of common analgesic (painkilling) drugs. Clockwise from upper-left these are; tramadol, ibuprofen, aspirin, paracetamol and metamizole. In each model atoms are represented as spheres and are colour- coded: carbon (black), hydrogen (white), oxygen (red), nitrogen (blue) and sulphur (yellow). Ibuprofen, aspirin and metamizole are non- steroidal anti-inflammatory drugs (NSAIDs). Paracetamol does not have any anti-inflammatory properties but is also anti-pyretic (it reduces fever). Tramadol is a synthetic opioid used to treat moderate to severe pain.