Nurofen plus tablets contain two active ingredients, ibuprofen and codeine phosphate. Both are pain-relieving medicines. Ibuprofen is a type of medicine called a non-steroidal anti-inflammatory drug (NSAID). It relieves pain and inflammation. Ibuprofen works by blocking the action of a substance in the body called cyclo-oxygenase. Cyclo-oxygenase is an enzyme involved in the production of various chemicals in the body. Some of these natural chemicals are known as prostaglandins. They are produced by cyclo-oxygenase in response to injury or certain diseases and conditions. Prostaglandins cause pain, swelling and inflammation. By blocking the action of cyclo-oxygenase, ibuprofen stops the production of these inflammatory prostaglandins and therefore reduces inflammation and pain. Codeine phosphate is a type of painkiller called an opioid. It is related to morphine but is not as strong. Opioid painkillers work by mimicking the action of naturally occurring pain-reducing chemicals called endorphins. Endorphins are found in the brain and spinal cord and reduce pain by combining with opioid receptors.