Paracetamol crystals, polarised light micrograph. Paracetamol is a painkiller (analgesic) drug widely used in over-the-counter medicines. It is thought to work by blocking the action of the enzyme COX-3, a cyclooxygenase, which is found in the central nervous system. Unlike aspirin and ibuprofen, which block all COX enzymes non- selectively, paracetamol does not have anti- inflammatory or antipyretic (fever-reducing) effects. It also does not irritate the stomach lining, as aspirin does.